My technique:
Use the "Print Screen" key, just hit it once with the picture site up. Open up Paintbrush, and go to Edit -> Paste. You should see the whole screenshot. Use the "box" tool (upper right) and highlight the picture. Go Edit -> Cut. Go File -> New, doesn't matter if you lose the old one. Go Edit -> Paste again, you should have your picture now. Make sure to save it as .jpg rather than .bmp, as .jpgs are much smaller. Now upload with your favorite site, I use photobucket. Voila!