Well to start, you have to have the image saved somewhere on the internet, most on here use photobucket, its free. You upload you pics, and then when you want to post one, you right lick on image and then pick copy image location. Then you post your message or signature, and when you get to where you want it, click on the little icon on the top of the compose message window that looks like a post card and then more cursor into the place that says http:// and right click again and choose paste. That will give you the picture that you want.
Hope that helps
John