I can help you with this! I tried for a week to find out how to do this and finally, with the help of someone on the DIS, got it.
You have to use a photo sharing site (I used
www.photobucket.com), register (free) and upload your photos to that site. Once you've done that, you copy and paste the "IMG" link below your photo(s) into your message and when you hit "Submit", your picture will appear.