I can't make any promises on this, as I work in VB, and am using a 5 year old reference book. So this may at least help you to move towards the answer.

It would seem that if you broadcast the WM_SETTINGCHANGE message ( &H1A ), this will hit all top level windows telling them a system-wide change has been made. I cannot be certain that this will include a wallpaper change, but this would seem to fit. If not, you may need to chase down some similar windows messages.

I hope this is helpful.