This isn't possible from within Skype for Mac, but it's easy using the Mac's snapshot feature: CMD-SHIFT-3 to capture the whole screen, or CMD-SHIFT-4 to select an area for capture. It's actually easier to do than to describe.
Entia non sunt multiplicanda praeter necessitatem.
Yesterday I could take a snapshot of my friend I was Skypeing with and this afternoon that facility is no longer available. I'm using Windows XP. What has changed and how can I take a snapshot. Thank you.