How to prompt your users to provide alt text for images?

We have an input field where the user can choose an image to upload, and if they select an image, they must provide a alternate text for the image so blind users can understand what the image is about.

What is the best way to describe the alt text field for a user who doesn't know what accessibility is and what the alt attribute is for?

I'm thinking of something on the lines of "please provide an alternative text for this image so that users with visual impairment can understand what the image is".