Google's John Mueller said on Twitter that the Fetch As Google feature within the Google Search Console is only meant to be used for the web index. This came up in regards to figuring out ways to speed up indexing old image URLs with new images. Glenn Gabe asked what if we used the Fetch as Google for images and then clicked the submit to index feature. John Mueller responded that it doesn't work with the image index, that it only works with the web index. John said, "Fetch as Google is only for the web index, it doesn't affect Image Search, new images on the pages get picked up faster though...