For your site favicon you need to create a favicon.ico file.
Try these sites to create a favicon from almost any image
favicon.co.uk dynamicdrive.com These two may only work on web favicon.cc degraeve.com (for it to work on all devices you must use 16px x 16px format).
Then just upload it to your root directory as favicon.ico
No other code is required for 99% of browsers.
And the icon will be used for all pages in your site.
Although you can add <link rel="icon" href="/favicon.ico" type="image/x-icon" /> To your _headtags If you wish
Using other images
You can also use the image tools to edit and resize any existing image on your site to 16px x16px and use it as a icon by adding this code to your _headtags file <link rel="icon" href="IMAGE_ADDRESS" type="image/x-icon" /> The same code can be used for favicons or images stored anywhere in your site or any other site