Q: some photographs show as black placeholders in imovie 10.1

You could try logging in to your computer under a different account and checking that iMovie behaves the same - this is to check whether the problem is connected with some third party software that runs in your normal account.

It is working fine for most users (including myself) there have been so many reports of this with version 10.1 and El Capitan that I suspect there might be a bug affecting certain types of input media.  I have not seen any clear solution and I recommend you send feedback to Apple at: http://www.apple.com/feedback/imovie.html  giving as much detail as possible on your computer configuration and details of the origin and format of affected files.

Geoff.

I have had the same issues and have spent 3+ hours on the phone with Apple Support.  They said that they are working on an update - but were not sure when that would be available.  It is HIGHLY frustrating!!!

Thanks for the info.  

I'm wondering if yesterday's El Capitan update (10.11.2) has solved this issue? 

Geoff.

an official support article was released with 10.1.1 addressing the issues with still images.

If still images are solid green or black in the iMovie Viewer - Apple Support