1. Find the picture on the web.
2. Right click on it, and select Properties.
3. When the information box comes up, highlight the Address(URL) part, and copy it.
4. When you post a message, click on the box labeled IMG.
5. When the dialog box comes up, paste the address in.
There are most likely other ways as well, but that should work, as it's what I've been doing. I'm not sure if the same process would work with a picture on your machine -- I always post stuff off of the net, as that works for me (using the "Images" section of Google has proved especially useful).