Q: Compare photos before and after edit

Press and hold the "m" key

All key board shortcuts are listed under the help menu

LN

When I press the "m" key I only hear a drumming sound, but nothing else happens. I use el Capitan.

The "M" key will only work, if you are currently in Edit mode.  were you?

I see now that you have to do this in the edit mode.!

Thank you.

You're welcome