Landscape with the Fall of Icarus by Pieter Bruegel the Elder (c. 1560)
The irony!
https://upload.wikimedia.org/wikipedia/commons/c/c7/Pieter_Bruegel_the_Elder_-_Landscape_with_the_Fall_of_Icarus_-_Brussels%2C_Royal_Museums_of_Fine_Arts_of_Belgium_-_Google_Arts_%26_Culture.jpg
Link to download at full resolution
https://gecdesigns.com/wallpapers/free/beautiful-fantasy-wallpaper-ultra-hd-wallpaper-4k-sr10012418
A black and white cool option.
Link: https://steamcommunity.com/sharedfiles/filedetails/?id=1817699663
I’m partial to this LHC pic, a wonder of the modern world on par with the pyramids at a minimum https://wallpaperscraft.com/download/hadron_collider_accelerator_particles_5630/3872x2592
NASA Hubble zoomed in on a faint edge of the Andromeda Galaxy, simultaneously showing a few stars from our own Milky Way, the dense sea of stars in Andromeda, and a few heavily redshifted distant galaxies behind
Full resolution download at:
https://photos.app.goo.gl/zmBcxtZjCMNp4hUy7
NASA Hubble deep fields, stitched together, brightened, and raised contrast
One of the best images of what the universe outside our galaxy looks like
Full resolution download at:
https://photos.app.goo.gl/dK8ovNgZcu95M2Vk9
Pluto:
Full resolution version available on Wikipedia: https://en.m.wikipedia.org/wiki/File:Pluto_in_True_Color_-_High-Res.jpg
the deeply strange "sombrero galaxy": full link
Taken by Hubble in '04.
The galaxy's hallmark is a brilliant white, bulbous core encircled by the thick dust lanes comprising the spiral structure of the galaxy.
At a relatively bright magnitude of +8, M104 is just beyond the limit of naked-eye visibility and is easily seen through small telescopes.,. The galaxy is 50,000 light-years across and is located 30 million light-years from Earth.
Whirlpool Galaxy and companion: link
Taken by Hubble in '05. located 25 million light years away.
The Whirlpool's most striking feature is its two curving arms, a hallmark of so-called grand-design spiral galaxies. Many spiral galaxies possess numerous, loosely shaped arms that make their spiral structure less pronounced. These arms serve an important purpose in spiral galaxies. They are star-formation factories, compressing hydrogen gas and creating clusters of new stars. In the Whirlpool, the assembly line begins with the dark clouds of gas on the inner edge, then moves to bright pink star-forming regions, and ends with the brilliant blue star clusters along the outer edge
this is stating the obvious but the wallpapers subreddit is pretty easy to browse/search.
I have friends that swear by wallpaper engine on steam—not free, but pretty cheap & will set you up ~forever.
if you like the kurzgesagt youtube channel, someone put together some wallpapers based on its animation (4k, although relying on AI upscaling it seems)
if you like fantasy-coded stuff, WotC has a site that draws from its endless well of Magic: the Gathering art for high quality wallpapers https://magic.wizards.com/en/news#wallpapers. (if you have no mtg connection this may not be as appealing)