If you already have an image file hosted on the Internet and ready to go, you just need to enclose the URL (path) to the image file in UBB code in your post as follows:
If you need a full tutorial on making a screenshot and getting it hosted on the Internet, there is one linked in the Siege sticky thread at the top.
Here is a copy of the link.
If you are looking for a much more abbreviated and less picture intense tutorial, I made one in that sticky as well, which does not require you to leave Stratics on an external link.
Linky.
-Skylark