I don't know if I'm right, but, as you don't use the scenarios of the Illusion game, where you got these girls from, I believe you put them in the potion you want, on a neutral color background I imagine, and then cut and put them them in the scenario you want, right?
I think that's what makes them strange, maybe it's the lighting issue where they don't seem to be in the same color and tone.
I believe that is it.
Do I have any suggestions? Maybe I think it's cool for you to talk to other developers and see if they give you tips on how to improve the quality of the images, leaving them in full HD or higher.
Maybe you can make the images bigger, with a resolution above 1280 x 720 and then compress them.
I think that's what I can say
doidera You're right about the figures not matching the lighting and tone of the backgrounds. The clearest example is the second image sample in the OP. The jungle has a soft overhead diffused light source while the character images are using a harder source with much less diffusion from the right and slightly up from the viewer position. Most of the sample images have similar lighting differences.
The tools for this are probably in whatever studio software you're using. It's a skill that isn't difficult, but it does take time both to learn and practice. I've seen similar issues in games by more experienced developers.