Hi Omikane and kirstendryburgh,
I just solved my image display problem, I may have a solution for you both. My most recent post was copied from an original draft. Rather than edit the draft, I copied it, re-wrote it and published the new version. Though I selected a featured image from the image library, none showed in the reader. I later went into that post and clicked on the ‘add media’ button. Then I clicked on ‘media library’. There is a drop down menu that gives you the option to view pictures ‘uploaded to this post’. When I clicked on that it showed zero images because although I had images in my post, they had all been copied across from the original draft and I hadn’t uploaded any new images. (This would also happen if you just used images that were already in the media library).
This is where the reader takes your images from (I’m guessing). If this is empty, you’ll need to upload the image again while in the edit mode for THAT post. You don’t need to insert it in your post. Just click out once you’ve uploaded it and click, ‘update’. And voila, it will appear in the reader.
Well, that’s what happened for me. I re-uploaded the featured image and suddenly it appeared in the reader. Sadly, I realised a bit late, so anyone who saw my post in the reader during the first few hours would have just seen text.
So, I actually believe you can control what images appear in the reader.