The 'Represent minimized windows by their screenshots' feature doesn't work correctly when the 'Show Desktop' docklet is used to minimize all running apps. It still makes their icons small and in the lower right corner of their space, but the space where the screenshot thumbnail should be is blank. Here's a screenshot -- Firefox and Thunderbird were minimized using the 'Show Desktop' docklet (which is the one all the way on the left, I gave it a new icon).