Good question, its another one of Google's little surprises that I'm unsure of its purpose or benefits.
Seems that some website images are not displaying the same way in Google. Being so, the website owner gets a notification for missmatched images and this can be fixed only in digging the actual code of the website, fixing, re-checking the image itself in Google. And if that's not enough you would have to request from Google to resubmit your site with the new images.
What bugs me is that nowhere in the notification there is an explanation of WHY such missmatch happens, WHICH images are exactly affected, or HOW can we check ourselves frequently if all our images are ok.
Oh, OK now I understand. But why would anyone do that?
Its a form of cloaking, basically tricking the GoogleBot that it links to a different content.
To tell you the truth I haven't heard of someone exploiting this aspect, its more of a manual penalty that Google applies and its definitely better not to ignore it. So once you get such a message in your Manual Actions section, you better fix the image and resubmit the site to Google through the Webmaster tool or you risk further penalties.
Panda, Penguin, Hummingbird, Penguine2.0 these all were Google's previous updations, so many numbers of websites got affected due to this updations and fall into bad penalties, now image mismatch is coming as goggles new updation, to keep websites safe get aware about this update.