How does Google rank images?

If the text is embedded as images, we may process the images with OCR algorithms to extract the text.

What happens with the images in PDF files?
A: Currently the images are not indexed. In order for us to index your images, you should create HTML pages for them. To increase the likelihood of us returning your images in our search results, please read the tips in our Help Center.

 

 

Source: https://webmasters.googleblog.com/2011/09/pdfs-in-google-search-results.html

Research: https://googleblog.blogspot.com/2008/10/picture-of-thousand-words.html