It's not possible to take screenshots of a desktop other than the one you're currently on.

[FAQ] Can I record the content of an inactive desktop?

Hi,

I work on oDesk, and I use oDesk Team application which takes screenshots of my work every few minutes. I've set up a virtual desktop which I called "WorkSpace", I assigned the application "oDesk Team" only on this desktop but the application takes screenshots of what ever desktop I'm on. If I am on "desktop 1" it will take the screenshot of "desktop 1" and not "WorkSpace" desktop.

The application appease on the system tray of all desktops.

How can I fixe that ? Thank you