I know that this should really be a fairly easy thing to do but I have been trying for ages to do it, gave up, then searched just about every where to find the answer to this:
How do I tell iTunes that I want to add the pre-loaded image (called theArtwork) as the artwork for the tracks just as you could rename them and add other info?
What you need to do is convert the file, where you loaded your image from, to PICT. Then read the PICT from byte 513 as picture, dropping the 512 byte header. You can use this picture data for the artwork. I cannot tell you how to do this as I am using Jaguar.