The first thing you have to do to post pictures is to find an image host. I have tried a few different programs but have never found one I am happy with. The two that I like the most are Yahoo Photos
Yahoo offers unlimited photo uploads but limits the viewable size of the photo and you cant link to the photo to show up directly in a post, you can only link to the yahoo site. Webshots is just the opposite. You can get a big viewable image and link the photo directly to the post. However there is a limited amount of space you can use and in order to upload multiple images at a time you have to download their software.
After you get you images uploaded you have two ways to post the image. The first method is to link to the site they are hosted on. To do this copy the hyperlink and go to the forum post page. Find the block labeled Instant UBB code and click on URL. A window will pop up and all you need to do is paste the link you posted into that window.
The Second method to post a image is to make it so it shows up directly in the post. To do this you need to first copy the URL of the remotely hosted image (can't do this in Yahoo). The easiest way to get the image URL is to right click on the image and select "Copy Image Location." Now go back to the forum post page and under the Instant UBB Code section select Image. Paste the URL you just copied into the box and your done.