De man wallch
:
--next Proceeds to next image (random image selection if this is selected) if process is running. --previous It proceeds to the previous image (the selected options doesn't matter, previous pictures are logged). --once It just changes the desktop background choosing from the list. It works both if the wallch is open or not. 2 and more pictures have to be selected for this to work.
Eu configurei atalhos de teclado para emitir o comando wallch --once
e ele funciona bem.