I'm rather fond of James Webb Space Telescope images, myself. It's nice sometimes to close out to desktop and have my breath taken away. Not sure if they have wallpaper-sized images; I like to grab the full-res ones and just look at a portion of the full image.
Like, I've used silhouettes of Politoed/Poliwag on colour before, and the shirt design from Yume Nikki (black/white/pink blocks) enough to be nerdy without being in your face.