Maintaining aspect ratio with min/max height/width?

Question

I have a gallery on my site where users can upload images.

I would like the images to sit in a div that maintains its height, the images should be no larger than 500

Answer 1

The only way that I know of to possibly accomplish this is by setting either the width or height to auto.

#gallery{
    height: 500px;

    img{
        max-height: 500px;
        width: auto;
    }
}