OS X already has support for Extensions. Check System Preferences. The relevant category here is "Actions". The system already provides a "Markup" action that p

OS X already has support for Extensions. Check System Preferences. The relevant category here is "Actions". The system already provides a "Markup" action that provides Preview.app's markup capabilities to other apps that embed images. On my system right now, Pixelmator also exposes one action called "Repair Tool". You can try this out by creating a new TextEdit document and dragging an image in there. In the upper-right corner of the image there's a dropdown button that shows you the available actions you can take. I certainly hope Photos.app supports Extensions. It would be a pretty obvious oversight not to do this.