Here's what you do:
Presuming you have your picture in .jpg format, find some hosting (I recommend ImageShack - it's free and you can add as many pictures as you like) and upload it. Take the bottom link and paste it into your message. Put [ IMG ] before and [ / IMG ] (without spaces, of course!) after the link and it should all work perfectly, unless their server is down or something. And that's all there is to it, pretty much.
HTH. 