User blog comment:Kirkburn/Technical Update: October 21, 2014/@comment-4522253-20141021205852/@comment-957747-20141021211741

A shared repository simply means that one (or more) wikias are pooling images from another. They can still upload new images as you'd expect, but they will have access to all the 'shared' images as if they were local. It's very similar to InstantCommons, but instead of pulling from, it pulls from the target wikia.