Shell – ImageMagick: multiple files conversion & saving with different name

I'd suggest a different approach, avoiding the possible word-splitting issues of ls

#!/bin/bash

shopt -s nullglob

for ext in jpg png gif; do 
  files=( *."$ext" )
  printf 'number of %s files: %d\n' "$ext" "${#files[@]}"

  # now we can loop over all the files having the current extension
  for f in "${files[@]}"; do
    # anything else you like with these files
    :
  done 

done

You can loop over the files array with any other commands you want to perform on the files of each particular extension.

More portably - or for shells that don't provide arrays explicitly - you could re-use the shell's positional parameter array i.e.

set -- *."$ext"

and then replace ${#files[@]} and ${files[@]} with $# and "$@"

It should be possible to strip the timestamps, using -define png:exclude-chunks=date,time; but there's apparently a bug in the PNG encoder.

If you can stage your modifications, one possibility would be to use identify's signature to only replace an existing image if the pixel contents of the image change:

mkdir out
convert -size 1x1 xc:white out/w.png
if [ ! -f w.png ]; then
    mv out/w.png w.png
else
    if [ "$(identify -verbose w.png | awk '/ignature/ { print $2 }')" != "$(identify -verbose out/w.png | awk '/ignature/ { print $2 }')" ]; then
        mv out/w.png w.png
    else
        rm out/w.png
    fi
fi