You have to type into google a website like "Tinypic" and upload a pic/screenshot onto that and it will give you the URL's for that image.
You need to copy the second URL down which is the "IMG Code for Forums & Message Boards"
Paste that into the post box and the image will pop up on the forums.
I know it sounds a bit complicated, but its better to be shown how to do this than told, in my opinion :/