I have found this MacWorld post showing an AppleScript that you can execute to force that.
I have my desktop images set to change every 30 minutes in random order. Sometimes the desktop will display an image that displayed earlier in the day, and I wanted a way to 'advance' the image easily. The following AppleScript will cause the desktop image to change, in whatever order you have set, each time it is run:
And this is the script:
property theSwitch : 0
if theSwitch = 0 then
tell application "System Events"
tell current desktop
set change interval to 1801.0
end tell
end tell
set theSwitch to 1
else
tell application "System Events"
tell current desktop
set change interval to 1800.0
end tell
end tell
set theSwitch to 0
end if
Please check the link and the comments for more similar or complete alternatives.
Note: I have not tested this.
UPDATE I'm sorry to hear that it didn't work for you, yet it does for me on Snow Leopard with the following script:
tell application "System Events"
tell current desktop
set initInterval to get change interval -- get the currrent display interval
set change interval to -1 -- force a change to happen right now
set change interval to initInterval -- change it back to the original display interval
end tell
end tell
WARNING
This assumes that you have
[X] "change picture ever x timesteps"
[X] Random Order
The above works for me. I ran it from the AppleScript Editor several times to make sure it was working and it indeed does change the Wallpaper.