Maintaining aspect ratio with min/max height/width?

The property you’re looking for is object-fit. This is one of Opera’s innovations, you can read more about it in their 2011 dev article on object-fit (yep, it’s been around that long). A few years ago, I wouldn’t have been able to recommend it to you, but caniuse shows that everyone else is starting to catch up:

• Opera 10.6-12.1 (-o- prefix)
• Chrome 31+
• Opera 19+
• Safari 7.1+
• iOS 8+
• Android 4.4+

http://caniuse.com/#search=object-fit

#gallery img {
    -o-object-fit: contain;
    object-fit: contain;
}

Using a value of contain will force the image to maintain its aspect ratio no matter what.

Alternately, you might want to use this instead:

#gallery img {
    -o-object-fit: cover;
    object-fit: cover;
    overflow: hidden;
}

http://sassmeister.com/gist/9b130efdae95bfa4338e