Sorry for misunderstanding,
Check out these instructions here: https://developers.facebook.com/docs/guides/appcenter/#uploading
Especially those:
- Don't put logos and text against the border of images.
- Make sure banner images fill out the frame completely (and that they leave no white space, no rounded edges or no borders).
- Round the edges of icons. We recommend that you use transparency and PNG, not GIF.
If your problem persist, Try another program. Personally, I use Adobe photoshop (Actions) to generate all graphics for my facebook apps, also there's tons of other programs that can help.