Here's an ImageMagick mogrify command I used to resize all "*.png" files in the current directory to a resolution of 534 pixels by 402 pixels:

mogrify -resize 534x402 "*" *.png

I thought I had to do this using a shell script, using the script to loop through all the PNG files, but that's not necessary, this one mogrify command does it all.

FWIW, all the image files started at the same resolution, around 800 pixels wide, that's why I chose this command to make them all this same size.