How to set the maximum width and height of an image in CSS while maintaining aspect ratio?

Question

css

So it goes.
It is necessary to somehow prohibit the image from changing its proportions, but how?

Answer 1

masscode.ru/index.php/k2/item/45-lifullsize

Answer 2

img{
max-height: 200px;
max-width: 200px;
heigth: auto;
width: auto;
}

Answer 3

background-size: cover;
https://jsfiddle.net/psr1919plus21/5khsp9qt/1/