Google added a description below the image (Caption). The description is taken from the title of the page in which it appears instead of the ALT tag.
When Google was asked what happens in case the title is unrelated to the image itself, a Google spokesperson replied:
"There is no change in ranking for this launch. We already use a variety of signals from the landing page in order to help deliver the most relevant results possible for users."
What does this means for you?
Google's change makes the page title as important as the ALT tag, which is the standard tag used to describe images. Therefore, the best way to work with images on a page that has several images, is to find common grounds between all images on the same page and use the most suitable page title.
If you have a single picture in a page, make sure that you write enough text on this page, otherwise you may experience a decrease in ranking. In addition, sometimes SEO plugins like Yoast are incorrectly configured to open every image in a new page instead of sending the file directly to the browser. If this issue exists on your site, it should be fixed.