See manage attachments or if you have a source where you can host pics, use the IMG tags for example
For this:
You [IMG]
http://img196.exs.cx/img196/1492/hpim10104mz.gif [/IMG]
Minus the the spaces between ] and h and the space between f and [.
Attaching an image is quite simple. Hit the reply button(The one that is in red) to get the proper reply screen, then scroll down, you will see a section that says manage attachments.