Turn OFF the Ambient light inside user params. Some people are more sensible than others to these quantizations. I only see that by zooming the picture. (I’ll see if float_framebuffer or srgb_framebuffer can fix that).
BTW, there are three lighting params you can turn OFF or ON: LIGHTS, SHINE and AMBIENT. And, for the last two, you can control intensity and size.
And about the retropie’s artworks. Are you loading using the auto preset? It rotates the game seamlessly (for this, your Core Option Vertical Mode should be turned off). Then, overriding the core, voilá, it loads the artwork automatically by loading a game. Better yet, it loads blazing fast. I’d like to integrate it with other great packs out there, it’s just that I don’t have much time to dedicate to it.
I wish Retroarch had a shortcut like M and N to load through the games in a folder the same way you can load the presets in a directory.