You can't do this with a picture on your computer i.e., it has to be from a picture published on the web on a site or in photobucket for example but if you see a pic you like, right click and it will bring up the properties box. Left click properties, when that box comes up left click hold down and highlight the link, right click to select copy and copy the link. Then post a reply to a post click the wee picture thingy icon delete the http bit and paste your link into it then post the reply.
It's easier than it seems but on second thoughts maybe best to stick with the photobucket thingy.