since you did the required combination to take the screenshot, go to paint, and click edit then paste.. after this save it as a png (recommened), or jpeg.
... now.. you need a hosting site, go to
imageshack.us to upload your picture.. click browse, and select the pic you want to use.. you should see upload somewhere at the bottom, when you click this button you will be sent to another page, and when you're there you should copy the code along with the uploaded picture (the one at the very top)...
when you want to show this picture paste the code with the uploaded picture here so you can see if it works... you can also show this picture in your sig. by going to control panel, edit signature, and scroll way to the bottom, and paste it thier..