fuzzy thumbnails on my main page

I am not sure if this would be a topic for Themes, but, i have tried this issue under at least 20 themes with the same result, so i kind of assume it may have something to do with my lack of knowledge on how exactly WordPress prepares the FEATURED IMAGES??

The issue:
I started this site – https://buy-acrylic-paintings-art.com
with lots of photos of fine art paintings on it.
I have to use one photo as featured image to get it onto the main home page.
The original photos are taken with a pretty expensive camera come out with pixel sizes of above 4000 and look reasonably crisp and sharp in full scale view – but once they end up on the main page as a featured image thumb of 290 pixels in width they are soooo ugly and fuzzy.
I used Photoshop to resize an original to exactly that thumbnail size of 290 px in width, so WordPress would have nothing to scale on it – but the same thing happens, once at the front page it is fuzzy again (while the 290px image in Photoshop was sharp and crisp)!!

I am totally baffled, how is this possible, the exact correct image size looks just okay in Photoshop, but once uploaded and installed as a featured image it is fuzzy, beyond me….

I tried a regular landscape jpeg photo – nullo problemo!
(the paintings are also all in jpeg format).

So, what is happening here, is there a certain BEST size of image for those WordPress “featured images”, or how else can i work around this to get at least the same quality on the website than i see it in Photoshop???