I already had the “Advanced Custom Fields” plugin installed
( https://www.advancedcustomfields.com/ )
so I just added a thumbnail field to my post type in the admin and the template itself. That way my portfolio listing template looked for the thumbnail image instead of the main display image it did at first.
I think the reason the crop-thumbnails plugin wasn’t working for me was because I was developing the site locally and there must have been permission or set up issues with it. I didn’t take the time to dig into it that thoroughly.