joveian: xnconvert is free for non-commercial use but is not open source.
joveian: Don't forget
gimp, which is also available on Windows and even has an official Windows Store version for anyone who cares about that (also macOS including ARM64). The installer lets you install it for everyone or just the current user.
To shrink an image go to 'File > Export As' then change the extension to .jpg. It will then popup a .jpg specific dialog box that has an option to preview the image, which will also show the file size. In the viewer, type 1 (or 'File > Zoom > 1:1') and you can check and uncheck the preview to compare. I like to first set the quality quite low to see where on the image the compression artifacts are most visible then watch those areas while increasing the quality until it looks ok.
You can resize the image from 'Image > Scale Image' and type R (or 'Tools > Selection Tools > Rectangle Select'), select a part of the image, and then crop with 'Image > Crop to Selection'. The menus are available with right click as well as at the top.
I'm currently playing Crystal Project. Not much going on in this image, just a penguin and a ibek hanging out in a tree.
What most surprises me about this whole shrinkage stuff is the amount you can shrink without loosing too much visual fidelity, i mean these 7 to 12 mb pictures shrunk to 488 kb and stil look more than okay....
makes you wonder if something similar can be reached for the moment you play games yourself like