It's simple. At the website click 'browse', find the file (picture) you want to show us, select it, then click "Host it!"
afterwards you'll have something similar to this: (nevermind the jew jitsu, heh.)
Copy the address at the bottom, and insert that into your post.
Since it's hosted on an image server, we can download and view it now. We can't do that when you have it pointing to a file on your hard drive.