In another thread (Placing An Image Next To A Block (UL) Without Changing Block Behavior - #42 by Chris77)
height auto is great for keeping an image looking normal (maintaining aspect ratio), but in this instance original look isn’t as nice as as-large-as-it-can-be look, mainly because small screen devices are mainly used in the upright rectangle position, giving an out of aspect ratio image a movie theatre look. Movie theatre look is better than too small look.
Chris