Show us some picture so we can see what is really a problem.
First of all, I'll mention, use old version of gimp. Gimp 2.6
It has png and jpg file which new version doesn't have.
Make sure your picture is 513x513 or 512x512.
Be aware that you need to see in that preset_(number) note(text document), which name and type(jpg or png) you need to use for your preset.