We recently moved from player cards to summary cards at the request of Twitter, and went live with the implementation a couple of days ago.
We’re now receiving complaints of the summary image not showing, both on the web and on the Twitter mobile app (so far only complaints from Android).
When testing the url with the validator tool, it correctly shows the image 100% of the time, but I can confirm the transient failures on both web and mobile myself.
The url in question is https://stipple.com/photos/52942473 . https://twitter.com/melchp/status/385394273354395648 is an example of a tweet that uses this url.
I’ve seen that others have a similar problem as well.