This is advice that Russ gave me.  I used it to post a picture from Maria but found that the message sometimes disappeared.  I hope you have better luck.
In the example he gives, I have replaced every / with the word slash, so you can see the text, otherwise it only brings up the picture.
David

"People basically do it at the moment by uploading their image to another website (generally a free image hosting provider).

A quick way would be to upload their image to this free image sharing site:

http://imageshack.us/

Then once they have the URL for the image, they can add it to their posts by adding the following text to their message:
[img]http:slash slash papiermache.co.uk slash images slash common slash logo.png[slash img]


That would include the PapierMache.co.uk Logo in their post. Basically, it's just putting the image URL between [img][/img] to make it show up."