How can I set it to overwrite images with uploads?

I’m setting up a site for a friend who knows nothing about html and I want him to be able to change the images on his site with no problem. I want to basically tell him to name the images specific names and then upload them, overwriting the old images so the new ones will just fall into place. But WP smartly is set to add a numeral to the end, preventing overwrites. How can I override that?