For a grid-style gallery you can have a container for each image with a fixed size (the maximum size you want an image to be) and the image itself can use max-w

For a grid-style gallery you can have a container for each image with a fixed size (the maximum size you want an image to be) and the image itself can use max-width and max-height with the same size of the container. If your aspect ratios are too varied, you might have to fiddle with the ideal size to get an optimal result, but it gets the job done. If it's another kind of gallery it could work if you accept those constraints. If you don't, then it's better to use width/height anyway.