Google's rationale for this innovation is:

"By adding more context around images, results can become much more useful...This week we’re adding captions to image results, showing you the title of the web page where each image is published. This extra piece of information gives you more context so you can easily find out what the image is about and whether the website would contain more relevant content for your needs."

In the screencap below, you can see the image captions in bold that Google has grabbed from the <TITLE> tag of the page and added to the existing page URLs accompanying each image: e.g., "Carambola: A fruit shaped like a st" under top-left photo.

So: a very actionable item: For optimal search ranking on Google mobile, make sure that for every page your <TITLE> tags, images and informational content are all mutually relevant. And that you don't have title tags like <TITLE>Homepage</TITLE> etc.