Two quick ideas on possible approaches:

1. Static page route. You can just write some Javascript to load the image from a file input in HTML, draw it resized to a canvas (based on an input slider or other input element), then save the canvas to an image. (There might even be simpler approaches if I wasn’t stupidly tired right now…) This can be done in a single file (HTML with embedded JS – and CSS if you want to style it a little) that you toss on any web server anywhere (e.g. Apache, nginx, whatever). Should work for JPEG, PNG, and probably WebP – maybe other regular image types too. Benefit: data never needs to leave your device.

2. Process on server route. Use Python with a simple web server library (I usually opt for tornado for stuff like this, but flask or cherrypy or similar would probably work). Set up a handler for e.g. an HTTP POST and either pass the image into a library like Pillow to resize it or shell out to ImageMagick as others have suggested. (If you want to do something clever with animated GIFs you could shell out to ffmpeg, but that’d be a fair bit trickier…) The image can be sent back as the response. Be careful about security if you take this route. Probably want some kind of login in front of it, and run it in a VM or some other secure environment – especially if you’re using AI to kludge it together…

Best of luck and let me know if you need any help. Will probably have some time this weekend if you can’t get it on your own. Happy hacking!

I would be happy with a FOSS desktop app I can install in linux too

On the command line, you can do this with ImageMagick (e.g. use the command convert once it’s installed).

With a (desktop) GUI, there’s a bunch of programs. GIMP is probably the most well known and has a ton of capabilities but is a bit complex. I use Kolourpaint as a quick-and-dirty “MS Paint”-like program for very simple tasks where I want a GUI.

If you want a simple web UI I’m sure there is one already, but I don’t know one specifically. It wouldn’t be too complicated to hack something up if all you need is a quick-and-dirty file input and percentage rescale or something like that. If you don’t get a better suggestion and don’t know how to make something like that yourself, let me know and I can write an example.

People have already covered most of the tools I typically use, but one I haven’t seen listed yet that is sometimes convenient is python3 -m http.server which runs a small web server that shares whatever is in the directory you launched it from. I’ve used that to download files onto my phone before when I didn’t have the right USB cables/adapters handy as well as for getting data out of VMs when I didn’t want to bother setting up something more complex.