I've seen this issue before, but if you let it go and don't worry about it, it will fix itself. Restarting the phone may clear them out. It seems like there is a hidden cache with resized versions of the pics for the tile and this cache gets updated at set intervals, possibly every 30, 60, 120 min???
Also, not sure if it's related, but possibly going into the pictures app and shuffling the backgrounds might speed up the cache update of tile pics.
Sent from my Lumia 900 using Board Express