Hit the Print Screen button on your keyboard (saves the image to your clipboard), then open an imaging program (Paint works just fine) and paste it there. (right click, paste OR control-V) Then save the file to your hard drive of course.
Press ALT+Print Screen to capture only the active window, then paste it into Paint or whatever you use.
Also, you can use a program called
Irfanview to capture the screen and automatically save it for you.
Then click 'Browse' ('Choose' if using Opera) when posting up there, find your image, and away you go!