From my reading of the documentation I don’t believe a featured image has to be selected in a post, however I have an error when I don’t have a featured image selected.
My site has been approved for both Summary and Summary Large Image card types. When either card type is selected but the post doesn’t have a featured image selected, the validator says “invalid url” and when I look at the code for a page the image tag is blank. For example here is the relevant meta code for my most recent post (http://trans-americas.com/blog/2014/02/vacation-rental-vs-hotel-room-quito-ecuador/):
However, when I have a selected featured image for a post the card validates and the code is fine
If I am misunderstanding this and a featured image needs to be selected, shouldn’t the card at least be pulling the fallback image I have entered into the plugin?