you need to go to photobucket.com, create an acount, upload your pictures to photobucket, copy the url from the picture. (the url will be directly under the pic). copy that into your message on here and it will show up.
in order to post pictures in here they have to be hosted first. photobucket it the best place, and its free.
Also, if you see a picture but don't want to download it you can right click on the image. Then copy and paste the location URL into the IMG code on here and it'll post for you.