Command Line to Convert Images for Stickers
The following tutorial is for macOS
brew update && brew install imagemagick
to your folder for the candidate images for stickers
convert all jpg & jpeg to png
magick mogrify -format png *.jpg magick mogrify -format png *.jpeg
for all the pngs, resize to be within 512x512
magick mogrify -resize 512x512 *.png
Afterwards, this folder of images meet the requirements for telegram stickers