I try to change my desktop wallpaper and I cannot choose a different folder from the one I chose at first...

When I first installed W7 I changed the wallpaper by right-clicking in the desktop and choosing Customize (maybe it's not correct because I use the italian version) and then Change Wallpaper in the bottom and finally I browsed for a folder with some images in and chose the wallpaper. When I do the same exact things now, the folder I select in the final pass is not selected: I mean its pictures are not shown and it doesn't appear in the drop-down list.

I can put a specific picture as a wallpaper by selecting it through Explorer, and some times (not always) the folder shows in the drop-down list.

Is there any possible solution to this?