You can select an icon (or upload it) and then fully customize it via this generator.

"The goal of this project is to offer a consolidated library of icons used in various libraries, window makers, themes and other applications. Over 10,000 unique icons in svg, png, xpm, ico, and icns formats. See homepage for details."

World leaders in icon design, we offer thousands of free icons, custom design services, software, royalty-free stock icons and much more.

Create a favicon out of existing images, which'll get converted and downscaled to a favicon size. Reasonably good downscale.