If the size was an issue, then why is it working for a few that I absolutely know has the art at 600x600px because I scaled it myself? I thought maybe PNG vs JPG, but they are all JPG. That's what makes the issue so frustrating: what is the cause? Not that a decent codec implementation should have these issues...
Just my 2c, could be a file (not pixel) size limit. I've had one player (don't remember the model) that had an undocumented 100KB file size limit. Since JPG compression rates vary, some artwork would show and others wouldn't. Took me days to figure it out back then.
Edit: on second thought, those who don't show could also be progressive JPGs, which are rarely supported on DAPs. Try to open one in a picture editor and save it as a non-progressive JPG.
Edited by james444 - 1/26/13 at 11:42pm