User Selected Header Images

I’m currently working on a theme and the owner has requested the ability for users to change their header image via an upload form. I vaguely recall seeing this in another theme before, though, I cannot recall its name.

How would I go about adding the functionality as a “theme option” in the WordPress Admin panel? For example, if a user logged in, they’d be able to go to the theme options panel for my theme > select an image > press upload > the image would automatically be updated on their site.

This seems fairly easy but I’m having trouble finding any information.