If your referring to an in-game screen shot
this guide can help you otherwise follow the steps below;
1) First things first, if you need to find the direct link to your image, this guide can help us do just that -
http://forums.d2jsp.org/topic.php?t=55042586&f=342) Now, let's try to find/use a site such as
https://imgur.com/ to upload your file, your going to click on "New Post" at the top of the page, and select "Upload images" from the drop box.
3) You then need to browse your files for the image your trying to upload, after your image is finished uploading your going to want to hover your mouse/cursor over the image and click get share links.
4) Then click copy the bbcoding (forums)
5) And we're done!
Code
[img]https://i.imgur.com/sqYiAso.jpg[/img]
Feel free to shoot me a pm if you have any other questions.