ImageMagick has a recipe for this in their documentation, see FavIcon Web Page Link Thumbnail
Essentially you run the following:
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
You can modify this to include larger resolutions as necessary and to change things like border, transparency settings etc.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…