WebP is a relativly new image format that provides lossless and lossy compression for images on the web. Google is an advocate for it since it helps speed up browsing and such. But do WebP images get indexed and ranked by Google Images?
The answer is yes.
Google's John Mueller said so on Twitter. He said "WebP images are fine for Image Search."
As proof, John said do a inurl:webp command, such as [flowers inurl:webp] and yes, Google has plenty of images indexed in that format.
Forum discussion at Twitter.