Since Facebook introduced the new profile layout people have been complaining about the bad jpeg compression of the cover image.
If you make sure that the following is met, you should be fine:
- Keep sizes at exactly 851 x 315
- JPEG (some use PNG-24 to keep the artefact / noise level low, has not made a big difference for me)
- Use sRGB as your colorspace
Some people seem to get better results, if they keep their images under 100KB. I have used images up to 400KB without any problems.
Keep those nice cover images coming!