If its sitting on your desktop, you need to go to
www.imageshack.us and upload it to their servers.
Once you do, a bunch of URLs will come up giving you links to the picture you just uploaded.
There are two that say "Thumbnail for forums" I would use the one that says, "Thumbnail for forums(1)":
Once you have that URL, copy it and paste it into the reply box for the thread, when you do that you will have something that looks like this:
If you want it to just show up, and not have a thumbnail that links to a picture, do this. After you uploaded the file to
www.imageshack.us, scroll to the bottom of the list of links and copy this one:
Then come back to the forums and in the reply box hit the button that looks like a mountain with a yellow background:
A window will pop up asking for the "text to be formatted":
Paste the link you copied into that and click OK, it will then insert the image into the thread, like this:
Sorry there is a lot to read, but images just make things easier. Good luck, I hope this helps.