I'm running Mint 19.1 on my laptop, and I don't know what changed, but sometimes images will just have this weird color palette.
On Firefox, it happens on some websites, but not others. Facebook is really bad about it, almost everything there is messed up. On Twitter, it only does it to the profile pictures.
It happens consistently with the Mint image viewer, but not at all with the desktop. Also, when I open a screenshot of a messed-up image in the image viewer, it sets it back to normal.
And here's what it looks like when I open the messed-up screenshot in the image viewer: