I had this problem too. Remember
Monique's Plan Outfit Everyday - Auto Thumbnail Refresh? The new feature seems to eliminates this mod. This is one of the mods I had installed and took out. I didn't put it back in but put the rest of the mods back and the constant every second changing thumbnail pics stopped but it still changes like when they wake up or just change clothes only.
The picture was changing like, every second with her mod in. So, I agree with ancienthighway, it seems to be a new feature.