An Image URL is the internet address which points to your image on the World Wide Web. Typacilly, once you've uploaded your image to a site on the internet, it will have URL address that will always point to your image on the particular website whether it is on MySpace, Facebook, Photobucket, Imageshack, Freindster, Hi5, Flickr and the list just goes on.
This is one of the easiest methods to bring one of your pictures over to our site, since you don't have to upload it again. Usually, sites will give you the URL address underneath the image or along with the "codes" for the image used to be placed on other websites.
Here are some examples of what an Image URL looks like: