Since the image file is created by the webserver, in order to delete it from the shell you must either be logged in as the owner (probably one of nobody, apache, or httpd depending on your config) or you must be root.
Alternatively you can write a quick php script that uses the unlink() php function to delete a file, then access that script from the web. i.e.
<?php
unlink(/path/to/file);
?>
-Vinny