Hi Muffles!
I assume that you are taking the picture from a website or have it stored on photobucket or something like that. All you need to do is find the url of the photo (e.g.
http://sonicyouth.com/alt-main/includes/reena.jpg), then click the icon that looks like a little yellow postcard in the message toolbar and type or paste the url in the box that appears, then click okay.
Basically, where you would usually have a link that starts with
url in this type of bracket:[] and ends with
/url in the same type of bracket, you'll have one that starts with
img in a [] bracket and ends with
/img in the same type.
Does that make any sense at all?
