What's the easiest way to free up disk space on my iMac with a huge photo library?
Assuming your NAS is always available and connected to your iMac, you can move both iPhoto and iTunes libraries to the NAS and use


symbolic links from ~/Pictures and ~/Music to fool the applications 
start the application with Option pressed to select the alternative location
iTunes: select alternative storage location in Options 
iPhoto: start by double-clicking moved library file