hey aine, here's how to do it:
If its on a website:
right click
select properties
highlight the address or URL, for example:
http://img19.photobucket.com/albums/v58/Dr...nnerproto07.jpg (thats our banner)
press control c or right click on it and say copy.
Then say add reply or new topic or whatever you want to do,
Clcik IMG button.
Paste the URL that you just copied in.
Click ok.
If its on your hard drive:
go to the bottom of the post adding screen bit
After the box for text, and the emoticons for the post, it has File Attachments
Browse for it, click ok, and it should come up as a thumbnail on your post.
Any questions?