If you prefer command line tool, you don't need to find a online to for convert images. Magick is THE powerful tool you deserve.
I'll collect a few scripts I used here as a note for myself, hope you can also find something helpful here.
make favicon
convert $1 -resize 256x256 -transparent white favicon-256.png # make 256x256 from source
convert favicon-256.png -resize 16x16 favicon-16.png
convert favicon-256.png -resize 32x32 favicon-32.png
convert favicon-256.png -resize 64x64 favicon-64.png
convert favicon-256.png -resize 128x128 favicon-128.png
convert favicon-16.png favicon-32.png favicon-64.png favicon-128.png favicon-256.png -colors 256 favicon.ico
