Blurry images – WordPress 4.6

Hello,

I found a lot of documentation and topics about blurry images,
but still can’t find the right solution or answer. (or I didn’t understand it…) ??

So, thing is,
I’ve perfectly sharp images, but once integrated in my website they look blurry.
Good example here : https://niyona.be/en/portfolios/for-cherry-on-the-bag/
(look at credits text on bottom images).

Usually I upload my images around 2000px, compressed at 60-80%, quite big I guess, but to make sure it’s fine for retina and big resolutions screens (maybe that’s already a mistake).
But I tried to upload 900px images, and the result is even worse.

What can I do?
I tried already an images resize optimers plugin, but nothing has changed.

Thanks a lot!