Q:

Is there a way to keep the original image aspect ration in amp-img tag?

In AMP HTML layout=responsive only keeps the aspect ratio I give to the image using the width and height attributes. Is there a way to keep aspect ratio of the image itself?

A:

There is infact a work around, if you give the amp-img a container div ( personally i do this dynamically in backend ) 
Then apply a height to the container, then you can use the css amp-img > img { object-fit: contain; }
Heres a full example I use across 25,000 pages to keep my page images with a correct scale.
<div class="image"><amp-img width="400" height="225" layout="fill" src="foo.jpg"></amp-img></div>
Keep note the width/height doesnt matter as width and height sets itself 100% of container when using layout="fill".

