In 2013 Google introduced new Image search results which were more visual. Apart from this update killing image referral traffic, it brought one more thing to the table: Google has put much more emphasis on the “visual” component.
Before that, ranking on top of image search results was no-brainer: You make sure your image file name and your image alternative text are your exact keywords and you are there. We’ve done it so many times..
Now, Google is putting much more emphasis on the visual variety of image search results.