When you reply to a post underneath the "email me whenever....blah blah blah" option there is a thing that says "posting how to" that will tell you.....
Let's say you find a pic you want on the internet...right click on it and go to properties....then copy and paste the url in the middle....if you do not have it online somewhere....I recommend photobucket account....it's free and shizzle. And once you upload it on there it will give you the url underneath.