Jason Snell has tried a few photo editing extensions on OS X El Capitan:

Like the built-in editing tools, you can actually stack multiple extensions while editing a photo, so you can combine third-party editing extensions with Apple’s own tools to get exactly the image that you want to see. However, each extension edits a “burned-in” version of your photo, so you can’t edit a photo with three extensions and then go back and turn off the first of the extensions. Instead, you’d need to revert back to the original photo (which is always retained by Photos) and start again from the beginning. You can also use the editing tools built in to Photos on images that have already been edited by an Extension, so you can really mix and match. You just don’t get the always-undoable, always-editable flexibility you get when you stick entirely to the native editing tools in Photos.

A good roundup, with some extensions I’ll have to try out.