The first thing you need to do is upload your image to a public server. You can try to use the members gallery on this site, but it hasn't been working right lately. I use http://photobucket.com
, there are many others available.
Once your pictures are uploaded to a website you can post them.
Open a posting window and click on the "Img*" box
Paste or type the address of the picture you want to post. Then click ."Close Tags
You can then click on "Preview" to see the picture in your post