There is a bug in Trigger where the calculation for the next time the wallpaper should be changed is determined incorrectly. The result is the wallpaper doesn't change when you expect it to (and perhaps won't for a very long time!).
This is happening because of the date/time locales set up on your machines. Basically the story is that if you don't have the dates set up on your machine in the American format mm/dd/yyyy (ie you have dd/mm/yyyy) then sometimes it will give you really weird results from date calculations because it confuses the month and the days.
I do have a good fix for this which I might be able to release in the not too distance future. I haven't looked at the Trigger code for a long time though so it should be an interesting trip down memory lane (if I can remember where it is :p).
If you do have this issue, email me and I will give you more specific details on how to fix it.
I am back after a wonderful holiday through asia. Now I just have to crank over the development machine and we will see what comes out. :)
Still on the holiday and still alive. Back in a couple of weeks.