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

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