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If you have suddenly noticed many of your images suddenly being dropped from the Google Image Search results, Google has been issuing manual action penalties for the little known “image mismatch” manual action.
While the manual action targets those who are displaying different images on the site to users when compared to the images shown in Google Image Search, it is also targeting images who use hotlink protection methods as well, as those will often substitute a different image when the script or program believes the image is being hotlinked, or not serve the image at all.
Gary Illyes from Google told me on Twitter “it’s confirmed” as a bug and the Google team is “working on it.”
This is not uncommon for the Google Search Console, formerly Google Webmaster Tools, reporting interfaces to have glitches. But this one is one of the more serious ones.