If you’re running AMP for fast loading pages on the mobile web (and, if you’re not, you should give it a try, it’s awesome!), you maybe got a notification in Search Console (AKA Google Webmaster Tools) that “Image size smaller than recommended size” for some or all of your AMP content.
Google Webmaster Tools
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How to Add Someone to your Google Search Console Account
I’ve had to explain this a few times recently, so probably there isn’t a good, clear explanation out there on Google, for this little corner of Google 🙂