Hello,I do not use image editing/modifying tools, so I cannot say if these solutions are good in your usecase, but there are at least three tools packaged in Debian:
- Converseen (GUI batch tool: conversion, resizing, etc...) https://www.makeuseof.com/batch-convert-resize-images-linux-using-converseen/ - FotoXX (GUI frontend to Imagemagick) - even Gthumb seems to be able to do this: https://www.maketecheasier.com/batch-resize-images-gthumb/
