Images are directly sourced from smashbros.com. Portraits can be found in each character's page. Icons are found on the Display by Series page.
Given a character image as a png, run the following commands in sequnce to generate a webp. These commands use both imagemagick and cwebp which is a webp converter by Google.
magick convert <input>.png -resize 1200x1200 -background transparent -gravity center -extent 1200x1200 <output>.png
cwebp.exe <input>.png -q 100 -o <output>.webp
For the resize/extent arguments, use 1200 for a character portrait and 200 for a character stock icon.
The output should be around 10KB for icons and < 300KB for portraits. Adjust the quality option in cwebp as needed.
The names of both the portrait and icon files must match. Names are in lowerCamelCase. Use the & symbol as it is used in Smash. For other special cases, follow the above convention and add a special case into the code.