generating correct favicons is trivial with image magick

favicon.sh

#! /bin/sh

# file : ~/bin/favicon
#
# usage: favicon foo.png

convert "$@" -resize 16x16 favicon-16.png
convert "$@" -resize 32x32 favicon-32.png
convert "$@" -resize 64x64 favicon-64.png
convert "$@" -resize 128x128 favicon-128.png
convert "$@" -resize 256x256 favicon-256.png
convert favicon-16.png favicon-32.png favicon-64.png favicon-128.png favicon-256.png -colors 256 favicon.ico
#convert favicon-16.png favicon.ico