Upload your images to a website that'll host them... like imageshack for example. Once they're uploaded - the site will give you a direct link URL which you can then use here.
Like this...
Photo taken at Womadelaide 2003. 10 second exposure taken around midnight. No photo trickery - the face was projected on to the tree with a speaker up in the branches. It yawned and muttered to people.
To attach an image to your post, when you're in the message editor screen, scroll down and click on "Manage Attachments". Then hit "browse" to browse your hard drive for the image(s), then click "upload" and post the message. The images will show up at the bottom of your post.