Lemme try this on my demo site
The code is(added star symbol so it wont convert to image):
PHP Code:
[url*=http://www.mysidiarpg.com/site/mys131/levelup.php?id=3][img*]http://www.mysidiarpg.com/site/mys131/siggy.php?id=3[/img][/url]
Works, so guess it is not the problem with the script.
Edit: It seems that your siggy file leads to an invalid page, which is weird. Did you upload that image through ACP?