Screencaps you can do either by holding down the Alt key while you press the PrtScn/SysRq button (it's usually up past the backspace bar, near the F keys), or to get the whole screen instead of just the one window, you can do Alt, Shift and PrtScn/SysRq.
Then, just paste them into an image program, save 'em, upload 'em somewhere like Photobucket, and they're ready to post.
