I got a lot of support requests from my Mac App Store apps describing drag & drop photo from Apple's app "Photos"(what a xxxx name they are using as an app!) to my app appears impossible under OS X El Capitan. That was ok in iPhoto ages.
The sandbox settings of my apps are all YES to these entitlements: com.apple.security.assets.movies.read-write com.apple.security.assets.pictures.read-write com.apple.security.files.user-selected.read-write
What I have to do is asking user drag & drop photos to their desktop first then drag & drop again to my app.
Anyone got this kind of problem for your third party apps and how can we fix it in OUR upgrade?
I created a sample project to demonstrate how you can do it here: