How to Keep Image Width Proportional to Height When Using Flex CSS Layout

Question

What's the simplest way to have the images in the following example maintain the original aspect ratio of the image?

I don't want to explicitly set the height and width of the image. I would like to be able to change the height of #app-banner-container and have the images resize accordingly. None of the solutions I found on StackOverflow seem to work.

<div id="app-banner-container">
    <img src="https://via.placeholder.com/150"/>
    <img src="https://via.placeholder.com/150"/>
</div>

#app-banner-container {
    display: flex;
    flex-direction: row;
    max-height: 75px;
}

img {
  margin-right: 5px;
}

JSFiddle: https://jsfiddle.net/9jhvrbwf/15/