Full-color photography isn't so efficient as .png, so you might want to use .jpg or try converting to .webp somehow*. Even the original photos from your camera might be better than what you're doing with paint which is likely just increasing data for no benefit.
If you stay with .png (makes more sense for screenshots/text) it'd help to reduce data
For .png especially, color reduction (crushing) if it doesn't destroy your image, which will greatly depend on content.
With G'MIC (usable via Krita, GIMP) a quick way to do this is a filter called ordered dithering which at the max (16 colors) takes it from ~16M possible colors to just 4096. You don't need to go this aggressive, but it's an option.
Posterization is a simple way as well.
Downscaling or cropping could help too, or for the future changing your camera settings down a size or two.
Also, image optimizers can help further (the one I use has multiple forms of compression, it works on JPGs too but I think it does more for PNGs. I don't think it's on Windows though). Some of this is stripping metadata, which might be a good idea if you have that (unless you could turn it off in settings too?).
* my instance does this by default, which is kind of annoying when I did optimize my image and everything (annoying because the lossy .webp has ugly blocks)