See http://www.docx4java.org/trac/docx4j/changeset/1684 for how to add an image to a slide.
Note this will need to be modified if you use anything earlier than today's nightly (which I'm still to create) or docx4j 2.7.1, since it is only from yesterday that BinaryPartAbstractImage.createImagePart has had a File signature. If you use byte, you'll be able to use this with earlier versions of pptx4j.