Session variables
If you are using callback routines, you can append URL encoded custom parameters to your uplink_url. Any custom parameters added will be available to your callback routine as GET parameters. e.g. webresizer_uplink_url = 'http://www.example.com/callback.php?var1%3Dsomething%26var2%3Dextra'
And/or set your own cookies to maintain session variables as needed. The API doesn't use cookies so there are no conflicts.