YES! exactly that! i was searching trough whole system for where is stored info of desktop background.... but couldn't find it and i think that it could be interesting, because you could use your theme wallpaper, no matter what theme is, and you don't need to replace manually wallpaper in config file for lockscreen thx for reading, and if it is doable, than ok, if not... oh well...
gconftool-2 -R /apps/osso/hildon-desktop/views