I see you found a solution fpr your self, just wanted to answer to your warining message:

system() has been disabled for security reasons

You or your host disabled the system function or even all execute function of php. This is done on a shared server to protect the server and clients in a better way.

Your workaround is nice. Images won't be stored and rendered everytime a user visits a post. This could be a problem on big boards...

You're right -- the re-rendering could be an issue for larger boards. In this case, you could build a new version of mimetex.cgi that uses the -DCACHEPATH=\"path/\" argument and still cache the images locally. This probably makes sense to do whenever the workaround is needed.

On that note, if you're building your own mimetex.cgi anyway, I'd recommend using the -DREFERER=\"domain\" argument too. This will add some level of security so that others cannot make use of the engine if it's located in a public area of your server.