There is a Win 2008 server with a few website, and sometimes an image conversion website consumes huge amounts of RAM. Is it generally safe (though not a good idea) to just kill that w3wp process (IIS Worker Process) in the Windows Task Manager or will it affect other w3wp processes (there is a separate Application Pool for each web)?
It is possible to configure the App Pool to be restarted based on memory usage. If you edit the advanced settings of the App Pool in the IIS manager then you can configure these values.
It is generally safe, yes. A better thing to do would be to stop and restart the application pool that the application is tied to. Either way, if there is another application in the same app pool you will restart that application as well.