A pixel size checkered pattern, not a checker board pattern.

b___b___b___
__w___w___w_
b___b___b___
__w___w___w_

One set (black and white), three transparent, or something like that. The alternating black and white should make it visible on solid black or white windows. Or maybe dark gray and bright gray instead of black and white.

You could also hang a little tab off the bottom corner that says "minimized". This combined with the grayed out thumbnail would let them know that the window was minimized(the obvious text) and not refreshing(the grayed out thumbnail).