Yep, the others are spot on.
The [img] tag tells the software to display the image located between the [img] and the [/img] tags.
Between the [img] and the [/img] codes the software expects a link in the form of
http://www.domain.com/image.jpeg. So to make an image what you should put is [img]address of your picture[/img].
You can't have any space between the various parts.
www.imageshack.us will also let you upload images. It's handy because once you've uploaded an image there, it generates code you can paste into vBulletin (ie. CPF) to display your picture.
Hope that helps.