How to resolve Low Image quality in Tablet and Mobile in android

I created two separate layout for phone and tablet layout-sw600dp this is my tablet layout folder. I created six button and i set background image for those button. The images i pasted in those folder drawable-hdpi drawable-mdpi drawable-xhdpi but i am getting poor image quality. I think i made some mistake in my code but i don't know exactly what. So please help me

my layout code

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://ift.tt/nIICcg"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    android:background="#ffffff"
    >


    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight=".5" >


        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"            
            android:layout_weight=".14"
            android:layout_gravity="center_vertical|center_horizontal" >


            <Button
                android:id="@+id/button1"
                android:layout_width="180dp"
                android:layout_height="180dp"
                android:layout_gravity="center_vertical|center_horizontal"
                android:background="@drawable/about" />

            <TextView
                android:id="@+id/textView1"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="10dp"
                android:gravity="center"
                android:text="ABOUT" />

        </LinearLayout>

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_weight=".15"
            android:layout_gravity="center_vertical|center_horizontal" >

            <Button
                android:id="@+id/button2"
                android:layout_width="180dp"
                android:layout_height="180dp"
                android:layout_gravity="center_vertical|center_horizontal"
                android:background="@drawable/ourprogram" />

            <TextView
                android:id="@+id/textView2"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                 android:layout_marginTop="10dp"
                android:gravity="center"
                android:text="OUR PROGRAMMES"/>

        </LinearLayout>



    </LinearLayout>


  <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight=".5" >

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"            
            android:layout_weight=".14"
            android:layout_gravity="center_vertical|center_horizontal" >

            <Button
                android:id="@+id/button3"
                android:layout_width="180dp"
                android:layout_height="180dp"
                android:layout_gravity="center_vertical|center_horizontal"
                android:background="@drawable/signin" />

            <TextView
                android:id="@+id/textView3"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                 android:layout_marginTop="10dp"
                android:gravity="center"
                android:text="SIGN  IN" />

        </LinearLayout>

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_weight=".15"
            android:layout_gravity="center_vertical|center_horizontal" >

            <Button
                android:id="@+id/button4"
                android:layout_width="180dp"
                android:layout_height="180dp"
                android:layout_gravity="center_vertical|center_horizontal"
                android:background="@drawable/testimonial" />

            <TextView
                android:id="@+id/textView4"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:gravity="center"
                 android:layout_marginTop="10dp"
                android:text="TESTIMONIALS" />

        </LinearLayout>


    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight=".5" >

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"            
            android:layout_weight=".14"
            android:layout_gravity="center_vertical|center_horizontal" >

            <Button
                android:id="@+id/button5"
                android:layout_width="180dp"
                android:layout_height="180dp"
                android:layout_gravity="center_vertical|center_horizontal"
                android:background="@drawable/enquiry" />

            <TextView
                android:id="@+id/textView5"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:gravity="center"
                 android:layout_marginTop="10dp"
                android:text="ENQUIRE NOW"/>

        </LinearLayout>

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_weight=".15"
            android:layout_gravity="center_vertical|center_horizontal" >

            <Button
                android:id="@+id/button6"
                android:layout_width="180dp"
                android:layout_height="180dp"
                android:layout_gravity="center_vertical|center_horizontal"
                android:background="@drawable/locateus" />

            <TextView
                android:id="@+id/textView6"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                 android:layout_marginTop="10dp"
                android:gravity="center"
                android:text="LOCATE US"/>            

        </LinearLayout>

    </LinearLayout>

      <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight=".5" >

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"            
            android:layout_weight=".14"
            android:layout_gravity="center_vertical|center_horizontal" >

            <ImageView
                android:id="@+id/facebook"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center_vertical|center_horizontal"
                android:src="@drawable/facebook" />


        </LinearLayout>

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_weight=".15"
            android:layout_gravity="center_vertical|center_horizontal" >

            <ImageView
                android:id="@+id/twitter"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center_vertical|center_horizontal"
                android:src="@drawable/twitter"/>

        </LinearLayout>

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_gravity="center_vertical|center_horizontal"
            android:layout_weight=".14" >

            <ImageView
                android:id="@+id/linkedin"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center_vertical|center_horizontal"
                android:src="@drawable/linkedin" />


        </LinearLayout>        

    </LinearLayout>

</LinearLayout>