Where to use {% header %} I was running a page speed test and it came up that I'm not leveraging browser caching for images and such. I was hoping to use the header tag to solve this. https://docs.craftcms.com/v3/dev/tags/header.html#app

Where does this go exactly? In the _layout?
This is something that is best done via server-side config. The reason is that when an image is requested, for instance, your Twig code that sets the header never gets run for that request.

You can see how it is done via Nginx here: Nginx-Craft

For Apache (don't use .htaccess pls) go here: html5 boilerplate apache