How to create responsive images with fixed height within tag?

I'm working on a website project where in the header section I have a grid of 6 images (2 rows with 3 images in each). It's not a problem to make them responsive (kinda "liquid") with max-width:100% and height:auto, however this website should be linked with some admin tool in the future, so the end user(s) could upload their own images.

Hence, I need to understand how keep these two rows of images responsive but at the same time give them a fixed height (in this case they should be 220px). When I crop the images and make them all equal in height (using Photoshop), everything works fine, but as soon as I use images with different height values, the grid starts to break. Is there any known workaround for this? Thanks in advance!