itch.io

There are some options to play with for lossy webp. Depending on your choice of compression, you might be better with jpeg. There is some overlap and discussion in the range of maybe 75 or some other constellation I do not quite remember. Maybe the colors look better in jpg. But I would try using the different presets for lossy webp first. My image app has picture, text, drawing and photo to chose, instead of default.

The second screenshot, the dark one in the bathing area with the walls. It is 2515kB.  As a placebo jpg with 100 it is 1249kB. A placebo webp with 100 is 612kB. An 85 webp is 172kB. 

You probably can go even lower. Just do a "blind" test. Put the compressed images along with the original in a folder and scroll through them in a way, so that you do not know wich is which and see, if or at what comprssion you can tell.

I did not notice any changes with the backgrounds. But curiously enough with the shade of purple of the text. But I have a very good display for colors and darkness. The text is not part of the image, only of the screenshot you used. Those image compressions are very good at compressing photorealistic things. Though I do not know which preset would be best for that. Photo or picture. Maybe it is photo. I am sure there are guides for this for specific cases, like dark images and shadows.

Point is, even at placebo setting you can go down to 25% ;-)