Sure can. Outside the game, find your picture (it'll be saved in the photos section), open it, press triangle to bring up the little menu, and select "set as wallpaper" - it's on the top row, 5th from the left.
But don't just take my
word for it...
Apologies for the awful quality, you can see the real quality of that shot in the impressions thread. And that's my Valkyria theme in the icons, if anyone's curious.