My guess is that someone put Gallium in an Aluminium pan. Similar to https://m.youtube.com/watch?v=IgXNwLoS-Hw

Nothing, it’s already ruined(!)

You’re looking for something to export the image at a lower resolution (resize) rather than something to compress the image. Any open source gallery/image editing app should be able to do this.

One example would be “image toolbox” on FDroid. I’ve not used it but it seems to have the feature you need