Is your selected background image is stored on the media card? And are you noticing any issues with the media card?
If the image is on the card, copy it over to device storage memory and set it to background from there, and see what happens. If the problem stops, you might test using another media card.
EDIT: Also see the following. http://supportforums.blackberry.com/...es/td-p/487316
Based on the info there, I think I might also try deleting all the .dat files.