Before you can use an image, indeed any file, on the internet, you need to upload it to a server.
There are three major options to choose from for hosting images - ImageShack (
http://www.imageshack.us), Photobucket (
https://www.photobucket.com) and Picdoodle (
http://www.picoodle.com/). There are a few others which can be found by performing a search for "
free image hosting", although in the past I have found them to sometimes be fairly unreliable.
Picdoodle and ImageShack don't require registration, although Photobucket does. I generally have found Photobucket the easiest to use and fastest, though, when compared with the sluggish speeds one gets at peak times with ImageShack. I've never used Picdoodle, so I can't really judge it very well. Sorry.
Google also now do a service called Picasa (
http://picasaweb.google.com/home?tab=mq) - I've never tried it, but if you have a Google account and wish to avoid multiple accounts with multiple passwords across the web, then this may be for you.
~:)