I wanted to add a mug shot of myself as this web site's icon. To create the icon, I started with an 8 megapixel photograph of myself that I had used for an ID card. Using the GIMP, I cropped it into a 2150x2150 pixel square, just barely chopping off the top of my head and the bottom of my chin. Then I scaled the image down to a 16x16 pixel square and made the background transparent. I saved it as
favicon.ico, with these icon details: 4bpp, 1-bit alpha, 16-slot palette. I copied the file to the root directory of my web site's local staging area, applied
chmod a+r to it, and uploaded it to the server.
AddType image/x-icon .ico
To make my web browser load the icon, I cleared the browser's cache, visited the web site, and marveled as my tiny mug shot appeared in the browser tab.
Here are some of the resources I consulted: