cheking user licence

[Anon]

We´ve just develope a PHP portal aplication for a client. We won´t to chek where this client is using it (it´s IP), couse we´ve sold him only one server licence.
Our client isn´t able to change the code.

We are loking at any easy way, as inserting a small gif on one page, so it will pull from our server for that image, every time a user visits the portal.

Any good ideas, tested or running ???

Thanks in advance,

JphpLover.