Are you speaking of the signature generator?
The reason it doesn't work is because normall CGI scripts are processed via the server first.
What my random image generator does is, when your computer tries to connect to get the image, the generator returns a "Location:" response, which is defined in the HTTP/1.1 spec to :
The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource.
So it basically redirects you to one of the random images I have the locations listed for in a text file.
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The reason this all works is because I've told my server to EXECUTE .png files. I have no problem with it doing this since I never host .png files on my home server, if I did, then I'd be up shit creek.
So, the reason it most likely does not work for you when you change the extension to .png, is because your server does not allow EXECUTION of .png and only allows execution of .cgi or .pl files.
That's why I suggested you use CHMOD if it's linux server, since I belive linux does not rely on extensions to determine if a file is executable, it relies on flags to do so. Those flags can be changed via CHMOD. You'd would want to rename your file to a .png, then do a chmod 755(I believe) on it.
~klinky