Display images with same height in a row without cropping

Hello,

like it is described here: https://presscustomizr.com/snippet/customizr-layout-examples/
I am trying to display multiple images in a row with the same height. But all my images have different sizes, so they appear on the page with different heights. I tried setting up a new image size of 400×600, but as I do not want to crop the images, that did not work well. I would prefer a responsive solution, where the images are proportionally scaled. How can I achive this?

Thank you for any ideas.