Linux

You can simplify the for loop by something like

#!/bin/bash
shopt -s globstar             #assuming bash
for i in images/**/*.gif 
do
     convert ..... $f
done

or, if you don't use bash,

find images/ -type f -name '*.gif' -print0 | xargs -0 convert .... '{}' 
# maybe xargs needs "-1" option to process only one file at a time

both versions call imagemagick's convert on all files ending in .gif in the images folder.

For testing, put an "echo" command in front of the "convert" .

You could also check out if there are Rust utilities which do what you want - you'd need to install these using "cargo install".

https://usage.imagemagick.org/resize/

Imagemagick is very powerful and easy to use, but be aware that the syntax can be a bit arcane. You will need to use mogrify to modify in-place, and make sure you tell it to only resize if it's larger, and to preserve the aspect ratio. If you just tell it "resize everything to 192x128" it will do exactly that, regardless of the size or shape of the input image.

imagemagik can do it. but I think you're going to need a for loop to iterate through subdirectories

ImageMagick does the job but can be slow. libvips is à faster alternative

Imagemagick with bash script. Lookup how to run a bash script recursively through sub folders

FFmpeg

Dont know if this is what you are looking for. I have never used it as I just write a quick python script, but I hope this helps.

https://github.com/Nenotriple/batch_resize_images

Take a look at XnView, it has batch capabilities of that kind, which you can set up through a graphical interface. Ships for linux too.