Title tags in Google image searches | Weboptim
Google has innovated the way it displays image search results, adding titles to stamp images.
The caption below the image is the same as the title of the page where the image is located, which means that these captions can be optimized by modifying the title tags.

Google says this helps users by adding additional content to the image. This of course depends on the title of the page.
Anyone involved in search engine optimisation knows that not all websites have well-written/chosen page titles. Also, if you value your ranking in Google image search, pay particular attention to optimising your title tags.
Google did not provide any details on the character limit, but it appears that captions are truncated after about 35 characters.
The captions have already appeared in Google search on iOS and Android, as well as in mobile browser image search results worldwide. In today's mobile world, it seems this update will not be present for desktop searches.
Source: searchenginejournal.com