Wordpress is messing up my images - can anyone help?

There is a certain kind of image that loses a lot of quality whenever I upload it to my wordpress site (running with a Divi Theme).

They are nearly-grayscale images, with a lot of white, and quite a lot of detail in the lighter greys. When I upload them they come out on the site kind of bleached out with much detail gone.

If I set the image in the webpage to link to the original attachment, clicking on it brings up the original image and this looks fine (same as it does on my computer before uploading to the website).

So obviously something is happening in the upload process where WP creates resized versions with some kind of compression applied. I want to stop it doing this! I've done quite a bit of googling and tried for example adding the code snippet suggested on this page but it didn't seem to help (unless I did something wrong).

I've attached screenshots of the original and a bleached-out version of a sample image. Although the difference might appear subtle to some it has quite a big effect on certain images, for the kind of thing I want to show.

Thanks if anyone can offer suggestions.

bleached out version that appears on webpage

original version as it should appear