Of course this is one of PRS+ most often used features!
See:
http://code.google.com/p/prs-plus/wi...#Standby_Image
and also check:
http://code.google.com/p/prs-plus/wiki/WhatsNewIn2_1_x
Goto Setting - PRS+Settings - Standby/Shutdown Image
(probably you will have to go to page 2 of PRS+Settings to access this)
You will have to put your photo(s) into /database/system/PRSPlus/wallpaper
The pictures are randomly chosen on standby.