convert $SOURCE_IMAGE -bordercolor white -border 0 \ \( -clone 0 -resize 16x16 \) \ \( -clone 0 -resize 32x32 \) \ \( -clone 0 -resize 48x48 \) \ \( -clone 0 -resize 64x64 \) \ -delete 0 -alpha off -colors 256 favicon.ico

via pfig’s gist