It took me one or two tries to get figure it out.
To post the pic, click the button marked "IMAGE" under "Instant UBB Code™" on the "Post A Reply" page. A user prompt window will pop up and you just paste the URL address in it. Just click "Preview Post" to see if, and how, the pic comes up in your post.
If the pic comes out too big (like it did with mine), open up the pic file in Paint and under the menu item "Image", adjust the pic size using "Stretch/Skew".
Hope this helps!