It's a question that can go in all sorts of directions:
You can change the URL of a given image if you can muck around with the server's configuration (eg. Apache's mod_rewrite) - useful if you're after nicer-looking URLs.
You can't to hide the URL completely - can't be done: if you want them to see the image at all you have to give them the URL. Once they have it they can use over again.
If the src of the img was "image.php" then chances are the image was coming from a database or another file somewhere. Using PHP's gd-lib API, you could then have PHP act as a relay: reading the image in and then spitting it back out. The URL that everyone then sees is that of the PHP script. The image can be somewhere else entirely (that PHP and the server can get to).