This time ask you this question:
Assuming we have this img ( generated by php ) linked in clients sites
<img src="http://mysite.com/genimg.php?id=2" alt=""/>
Is Possible to get back from the client location some infos!? ( es. client url )
PS:
You can figure out everything from $_SERVER global variable in your genimg.php, just as a normal script would.
For example, use $_SERVER['REMOTE_HOST'] to determine the host name from which the user is viewing the current page (in your case, loading the image). See more details in the manual.