The 2.4.1 patch in Diablo 3 added a lot of new cosmetic items, and this guide will teach you where you can get your hands on the Rainbow Portrait Frame. The portrait frame drops from a unique mob called Sir William. He is located in Whimsyshire, and make sure you do not confuse it with Whimsydale (in which he cannot spawn).
To get to Whimsyshire (also known as the pony level) you need to use a Staff of Herding. This item can be annoying to get, but check out our Whimsyshire guide in order to learn how to farm the items for the staff.
With this item in your inventory head to The Old Ruins waupoint and head towards the Old Tristram Road. After a little walk you find the rift in the ground and the Ghost of the Cow King will spawn with a quest indicator above his head. Talk to him and you can enter Whimsyshire.
Once you are in Whimsyshire start to search for Sir William. He can spawn as a unique mob, but he is really rare, so be prepared to search for him for a while before you find him. Once you find him, he will have a 100 % drop chance on the Rainbow Portrait Frame!
And once again, make sure not to confuse Whimsyshire with Whimsydale, where the latter one opens from killing Rainbow Goblins. These can not contain Sir William and the portrait frame, but can spawn Princess Lilian which gives you a set of wings!