Find an upload service (www.imageshack.us is popular).
In the box on the right, click "Browse."
A box will pop up. Navigate to the file you wish to upload for people to see.
Double-click it, then click "Host It!"
The bottom-most box displays just a link. Copy that, and paste it where ever you want to post the image. It will then show up as a link.
If you want it to show up as a picture in your post, on these forums, wrap the link in [ img ]link[ /img ] tags, without the spaces.