This for selecting single/multiple wallpapers
Right click in the desktop ->
Personalize ->
Browse (go to picture folder) ->
Select the picture ->
Save changes.
............
Could u explain your problems, Is it missing only when u turn windows on ? is it change to other picture or it just becoming black wall ?
Have u tried installing windows update for service patch/stability fix....... (keeping your windows updated could fix some/lots issue.....)