Image placeholders are broken with Sketch 47
![populator_sketch47](https://user-images.githubusercontent.com/1380294/31469227-e2b5c132-ae95-11e7-89aa-f7b1a5d349f1.gif)

Image placeholders aren't getting updated or replaced with images at all. 

Plus they are causing other placeholders to break. 

I tested this with one of your demo presets. 