First, you have to upload them to a site (I use my web page space from my ISP - a lot of folks use Photobucket). Then you need to get the URL for the photo...right click on the photo and choose 'Copy Image Location' (that is what it is in Netscape...other browsers may be slightly different, but similar in concept). When typing in your message, either on the icon above the smileys that looks like a portrait or simply type img with square brackets around it. Paste your image location in, and follow that with /img in square brackets.
So, it should look something like:
{img}http://www.wherever.com/mypicture.jpg{/img} and replace the curly braces with [ and ].
Hope this helps you out.