This is how to create a multi-resolution favicon with a transparent background using Imagemagick. Imagemagick has some documentation that gets us most of the way there, but we need to tweak the commands slightly to preserve the transparent background.
Requirements:
- Your PNG has a transparent background
- Your PNG has a square resolution that is at least 64x64 pixels.
Here is the command:
convert logo.png \
\( -clone 0 -resize 16x16 \) \
\( -clone 0 -resize 32x32 \) \
\( -clone 0 -resize 48x48 \) \
\( -clone 0 -resize 64x64 \) \
-delete 0 -alpha on favicon.ico