I couldn't find this issue raised before - apologies if it has.
Using Win 8 (64 bit) and DF Pro 5.0.
I have a dual monitor setup, where the primary is on the right and the secondary is on the left. They are running different resolutions and have different images set as the wallpapers.
This is how it displays when setup correctly:
http://imageshack.us/photo/my-images/407/dual1.jpg/
If I then disable the secondary monitor (via Win+P > PC screen only), the wallpaper of the right/primary monitor will change to that of the left/secondary, eg:
http://imageshack.us/photo/my-images/546/single1.jpg/
If I then go into the DF wallpaper settings and just hit OK/Apply, the wallpaper will refresh to display correctly, eg:
http://imageshack.us/photo/my-images/811/single2d.jpg/
Similarly, when I go back the other way (Win+P > Extend) from here, the wallpapers both get set to the right/primary one, eg:
http://imageshack.us/photo/my-images/12/dual2j.jpg/
I've only just made the switch to Win 8, but I've been using DF for a while now - never had this problem in Win 7. I didn't test it, but I'm guessing the problem only exists when the monitors are arranged as 2 1 rather than 1 2.
I also saw that other thread about the noticeable JPG compression on the wallpapers that DF generates.. just like to say that I'd love to see that one fixed soon too