You have to copy the image's url.. in otherwords it needs to be hosted on the internet somewhere. So like Tweek said, Tinypic is good or you could use something else. Once you have the link, copy it into the thread, then at the beginning and end of the link, put [img] tags. for example:
[/img] ... except no spaces before or after the URL, I had to put those in to show you though.
For this picture scroll down slowly for maximum lolz