How to manually trigger background image change?
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 &quot;System Events&quot;
    tell current desktop
      set change interval to 1801.0
    end tell
  end tell
  set theSwitch to 1
else
  tell application &quot;System Events&quot;
    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 &quot;System Events&quot;
    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] &quot;change picture ever x timesteps&quot;
[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.