Currently when one wants to set a custom background he is only presented with a file picker. It is not possible to select pictures from the wallpapers gallery.
This is really bad from user perspective, because one can use GHNS to get new wallpapers, but they are stored somewhere (nobody knows), so one cannot select them as a background.

Therefore I suggest to present two options upon click on set background image:
- Choose from Gallery... 
- Load from File...
(and Clear)

This is also how it is handled in the select avatar option of the user manager KCM.

Reproducible: Always