Instead of just writing a help note "Please upload 200x200px only" (that rarely gets honored),
Please let us specify a Width + Height of the image to be uploaded in the CMS collection's item.
So that when the collaborator uploads his image, he's constrained by a Crop tool dialog, that force him to crop and position their image properly.
Think about Facebook / Twitter's Profile Image "Crop Tool" dialog.
I believe Webflow would be the first in the world to create such a playful image feature in a CMS!
(that's customizable by the admin for each collection item).
Want to push it a little further?
Let the Collaborators play with CSS Image filters as well! (Boy that would be amazing)