Apache FOP: Image not found after packing resources into jar

I have problems with my Apache FOP application.

Everything worked fine, until I started to pack external files (xsl, ttf, ...) from an external working directory into the jar file of the application. Since then, no images are loaded anymore and I get the following error:

FOUserAgent - Image not available. URI: data:image/jpeg;base64,.... 
Reason: org.apache.xmlgraphics.image.loader.ImageException: The file format is 
not supported. No ImagePreloader found for data:image/jpeg;base64,...(No context info available)
org.apache.xmlgraphics.image.loader.ImageException: The file format is not supported. No ImagePreloader found for data:image/jpeg;base64

In the xsl file, the images are embedded with:

<fo:external-graphic xsl:use-attribute-sets="helpicon" fox:alt-text="helpicon" src="url('data:image/jpg;base64,....')"></fo:external-graphic>

Why are images not working anymore, when I pack the files into the jar? The images are embedded as a base64 string into the xsl file, so the image must be there...

Thanks :)