I am having this same issue. I have checked the link in validator.ampproject.org and it is failing.
My webpage link is: https://onsumo.com/happy-birthday-wishes/amp/
Though I’m facing the issue on every page of my website’s amp posts. But let’s take the above one for instance!
AMPforWP plugin is not fetching my website OnSumo.com in image src “amp-img” tags.
(You can check above link in validator.ampproject.org and see what I mean to say) ??
Please help!
Thanks!