But, if you are talking about the wallpaper, it's because Skinstudio doesn't delete successfully the wallpaper= line
Same problem in skinstudio for the rollup size 
Thanks a lot Johanne, i'll try to do the with a text editor
Edit: I'm not talking about Wallpaper, but about Explorer backgrounds. I Just edited the .uis file, removed sections relative to explorer_back_active.bmp, explorer_back_active_sorted.bmp, explorer_back_inactive.bmp, explorer_back_inactive_sorted.bmp, but it doesn't change anything, and option in WindowBlinds to apply an Explorer Background still don't work with that skin 