Somehow Apples cloud based photo system got enabled on my iPhone. Which also meant that the delete button in the OS X application “Image capture” got grayed out.
To re-enable it had to turn off iCloud Photo Library and My Photo stream via Settings -> Photos & Camera
And then I had to reboot the phone (running IOS 8.1.3)
I am old enough to remember that any change on a Windows machine last century also required a reboot.