How to Upgrade Memory in a 27-inch iMac (Late 2012-2013) iMac13,2 and iMac14,2 - YouTube
A-Tech 8GB Kit (2x4GB) RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com
Memory upgrade: Apple iMac 2012/2013/2014 (Howto)
CMS 8GB (2X4GB) DDR3 12800 1600MHz Non ECC SODIMM Memory Ram Upgrade Compatible with Apple® Mac Mini Core I5 2.5 (Late 2012) - A24 at Amazon.com
Review: 21.5-inch 2012 iMac takes two steps forward, one step back | Ars Technica
late 2012 - 9to5Mac
Install memory in an iMac - Apple Support
Memory Upgrades for iMac 27-Inch (2012 - Mid 2015)
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET