I’m not trying to be a wise-acher here, but have you considered that Gimp is doing something to the images that the Sansa doesn’t like?
All I can tell you is that I’ve not had one single issue with album art with my Sansas since I started using the freeware program called Easy Thumbnails
I instuct the program to resize the input jpgs to 275x275 (a good setting for all my Sansas and non-Sansas alike) and just let Easy Thumbnails do the work.
I usually embed my newly created thumbnails into the files itself using MP3Tag while keeping the folder.jpg image in the album directory as well.
With the above procedures, I have yet to have an image that didn’t display properly.
