Encontrado no site imagemagick.org :
convert image.png -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