jpeg is already handled as metadata. Your file just probably don't use the expected mapping Well i played around and it seems like JPEG's are displayed when several other ID3 Tags are filled. I tried with purchased M4A files: audio track with JPEG and ID3 tags: cover displayed i removed the ID3 tag...
When playing M4A files with embedded JPEG cover art, the cover does not show up during play or in the list.
However GIF files seem to be supported and displayed.
Please add support for JPEG files too, like for mp3.