I think that the size constraints are for the image to be under 300x300 pixels.
I had this happen with every DAP I owned to date, including some smartphones. Very large artwork might be displayed on some, but will make the DAP lag just because it was not deisgned to process the image files at a much larger resolution (CPU limitations)
Yeah I never thought about it much until some weren't showing up so I resized the art and it's fine now. The last thing this DAP needs is more lag!