This might have been posted before but i did not look through all the posts....
If you do not care about the extra load on your server you can make a .htaccess file with this code:
IndexIgnore *
<Limit GET POST >
order allow,deny
allow from all
</Limit>
RewriteEngine on
RewriteRule signature.jpg /forum/signature.php [T=application/x-httpd-cgi]
Then you would link to signature.jpg from your root folder of your domain ane apache would run the PHP file to get the updated signature.
Cheers