If the pic is on your computer, then you simply click on the "Browse" button on the preview screen after you write the post. Then you have to find the pic on your computer and dbl click it. Then submit the post. If it's too large, then it will tell you that (and if the software doesn't Glen will! /forum/images/graemlins/grin.gif).
If you want it embedded like Bevis did, then you link to it on a website somewhere by clicking the "Image" botton on the Instant UBB Code box.(next to the smileys). There are other ways too, but these are the usual ways.