Have a look at
this thread in the "Board Related Issues" section.
In a nutshell, you have to post the photo to the web somewhere and link to it. There's a photo gallery on this site, or you can use Flickr or any of the dozens of other photo sharing sites.
To link an image, you need its URL. The quick way to get it (versus the whole page), is to right click on it and select "properties". Copy the address. When you are composing your message, click on the "Img" button just below the subject line, paste the address, and then click the "Img" button again (it will appear as "Img*" the second time).
To make sure you've done it right, click "Preview" before "Submit".
BB