Upload a photo to a post

Hello,

I created a custom post with custom fields. One is the thumbmail.

My issue is that the user cannot upload the photo if he/she is not at least an author. Is it normal?

If I set the the user as author, he/she can then edit the uploaded image and then access to the backend of the site.

I want the user to not enter the backend and only upload the photo without accessing the media library.

Is it possible?

Thank you.