Well I dunno about profiles, but in a message...
the pic has to be under a certain size, and has to be one of the listed formats. People have reported problems when previewing a post into which they've inserted a picture - the pic doesn't always show. I used to have immense problems getting a pic into a post but I might now have the hang of it.
Regarding profiles - I am guessing (but someone who knows will be along soon) that it needs to be hosted on a webserver somewhere, rather than resident on your local drive. Then you would be able to load it in your browser by typing it's web address (like you say,
http://www.....)
o forst you need a webserver, or a good frienship with someone else that has one and who will host your picture.
Phew, that was a lot of typing consideing I'm probably wrong!