First, while in your game, hit the Print Scr (Print Screen) button. If you can't find it, it is located next to the F12 Key. After you press it, you can go to a free image hosting site, such as
photobucket.com.
To find the image, go through your folders. It should be something like this:
C://Program Files/Diablo II/ and all your screenshots are in there.
I hope this helps.
3nodding