JP,
You need to use an image hosting site such as photobucket, and to upload from there.
Thus, using my photbucket account:
1. Click on the insert image icon in the message tool bar (one in from the speech bubble on the very right hand side of the bottom row of icons)
2. When the script prompt box appears, enter the URL for your image - on photobucket, and I assume other sites, you simply cut and paste the URL link into the line in the script prompt box - in the case below, I'm pasting in the following
http://img.photobucket.com/albums/v4...-11-03067b.jpg
3. Then press 'OK'
4. Et voila!
I apologising for lowering the tone with an F3, I've not yet uploaded the shot of the 54 Sqn Jag refuelling that I have...
Photobucket is found at
http://photobucket.com/