I gave you an incomplete answer.
Flexbox is a way to align, order and distribute containers in space. An image with no dimensions won’t be resized by Flexbox itself.
So to achieve what you want without giving dimensions to anything, you need to remove the class on your image, put each image in a div and put the image class to the divs. Then it works as expected: both divs are 50% and the images inside resize automatically.
http://vincent.polenordstudio.fr/snap/h3aar.jpg