there are two ways
1) when you press prnt scrn (screen shot button) in game, it will automatically save a copy of the image in your d2 folder. You can then go to an imagehost such as tinypic.com or imageshack.com and find the image in your d2 folder then upload it, you will get a image code which you then put image brackets around when you post it like this: [img]your image url[/img]
2) after you take the screenshot, exit out of d2 and open up paint, then press ctrl + v to paste your image in, you can then save it to your desktop or wherever and upload it to an image host etc etc