How to capture an image of desktop and send it in an isapi dll?

hi,
I need an extension isapp to capture an image of current desktop and send it
to user. CWnd::GetDesktopWindow() works correcty to capture an image of
desktop on a usual application. But when I use it in an isapi dll , it reads
a black bmp, possibly becasue it runs on service space not interactive
space. how can I solve this problem?

thanks a lot,
Behzad