Something similar happens to me. I have had to remove the thumbnails from some fedds. Even with https pages. It gives the paradox that when inspecting the page and going to “console” it indicates to me that the problem is in images with http, but turns out that you go to the web in question and make cilc in the image and it appears with https. Which I do not understand.
For example. When I inspect my page because it indicates not secure, I point out that the problem is in some images, for example:
Http://alicantebdsm.files.wordpress.com/2017/06/18920711_1835783736743156_2718523220302829651_n.jpg
But if you open it in a new tab it appears:
Https://alicantebdsm.files.wordpress.com/2017/06/18920711_1835783736743156_2718523220302829651_n.jpg
If you go to the web in question and click on the image it also appears as https
