Adding images in CKEditor
I forgot to post an answer before, but am prompted to do so as someone has a similar problem.

I needed to set directory 755 and files 644 for wp-content/plugins/civicrm/civicrm/packages/kcfinder because the WP distribution didn't set permissions that worked in my hosting environment.

But since CiviCRM 5.2.2 this wasn't necessary.