When you upload photos to the internet, sites often reduce quality to make the page load faster.
Here I’ll upload the same original photo in three resolutions to see if the FINAL displayed image improves.
I can see a big difference between the original being 5472 as opposed to 640 pixels wide.
But the 1280 pixel version is pretty close to the 5472. When I checked, WordPress display default had reduced both the top two: 885 pixels wide.
IF I link to the original photos, however, someone keen enough to click could get a copy of the highest resolution available.
Some recommend I upload at about 1280 pixels. 80Kb-100Kb file size, at most.
If and when I switch to a theme that allows full-width images, WordPress recommends those be uploaded as originals with at least 2000 x 1200 pixels.
I’ll keep experimenting.