Keeping photos high resolution

I am having an issue getting my product photos looking nice and crisp. Is there a way I can ensure the photos will always look crisp at any size? I am using jpegs at original width of 800px at 72ppi.

After working with .jpg's for years, I create most all my images now to .PNG format,
to make them look ok in all sizes, 'shrinked' by OC. And I then use this fabulous
Online Tool, to compress them to it's best, either in bulk, or then one by one :
https://www.giftofspeed.com/png-compressor/
I 'dimension' images in the 'max-size'-resolution, as set in the admin, to so
present it in it's best quality, in large/full size. The image shown below has been
shrinked and then slightly sharpened again, but it's more made, to show you,
that even quite large Images can be compressed, to please Testsites like GTMetrix
like shown on the Site Main Page:
http://www.bigmax.ch/shop/
Just figured ...
Ernie
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PS: Just note, not all images look best, I just used, what I found and had available ...
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You really should be using webp format these days which Cloudflare will do for you if you don't want to modify Apache (and Cloudfare will do it on the CDN too, hard to argue not to have them just take care of all of it.) OC will do a very slight compression when it resizes images, my eyes are too poor to see the difference, but you can take the slight compression out of the PHP code if you wish.

Well, I just made some tests, by use of the ~394 KB large PNG Image shown above,
and after changing it to WEBP online, it shrinked to ~360 KB in Data Size. But as
compressed PNG File, it's size is only ~130 KB. And such sure does not make a
WEBP File Pageload speedier than by use of compressed PNG Files, as I assume ...
And I can't even upload it here, it's not accepted as Image Format.
Ernie

Thanks for your most helpful suggestions. I think making them 72ppi in the first place was a major error.