Automator can't find bash command

Question

I'm trying to make an action on Automator that recursively batch processes PNG images using bash shell script with pngquant

I have done this on terminal and got no problem, but now I want to make a finder action using Automator.

This is the shell script code (receives a directory input):

directory="$1"
cd $directory
find . -name '*.png' -exec pngquant --ext .png --force 256 {} \;

I'm having trouble with pngquant as it says:

pngquant: command not found