Doesn't Craft do something similar?
Yes. It's the feature I like most about Craft. But I stopped using Craft because of problems with Sketch updates, and I thought I'd write my own... Craft puts the same image in every symbol which isn't what I was after. My plugin gives you a random image for each selected symbol or shape. Unlike Craft, my version doesn't allow you to target specific layers in the symbol, instead it just swaps the largest image in the symbol.
If you're running version 1.1.1 of the plugin and you're wondering why it doesn't do anything... Something went wrong with the publishing process. Sorry about that! Please update to version 1.1.2. All fixed now :)