SlackBuildsOrg/graphics/png2ico

PNG2ico converts PNG files to Windows icon resource files. If you're
looking for a program to create a favicon.ico for your website, look
no further.

The program is extremely simple to use. To create a favicon.ico that
contains a logo in the resolutions 16x16 and 32x32 (an icon can contain
multiple images of different sizes, but a single 16x16 image is enough
for a favicon), you would use a command like the following:

png2ico favicon.ico logo16x16.png logo32x32.png