First off, I appreciate your plugin. Yoast SEO seemingly picks a random image from the post whereas the featured image, like you use, is a much better choice.
I'm having difficulty getting a large enough image for the summary_large_image card. I'm not sure if it's even related, but no matter what values I enter into the width/height under "Photo Card", the plugin always makes twitter:image be one that's something like 300 x 225 if it's a wide image or 225 x 300 if it's a tall one. -- The wide one satisfies the minimum Twitter requirements, but the tall one does not.
From https://dev.twitter.com/docs/cards/large-image-summary-card :
This Card requires an image with minimum width of 280, and a minimum height of 150
A 225 wide image is less than 280.
I'd prefer it just link to the "large" size image (and Twitter could resize it down).
Or you could add an option to select the image size (such a "medium" "large" or custom set ones like "_800wide" or whatever each person has created in their settings, like when inserting an image into a post).
That way, it fills the whole column when someone views the twitter post.
At the bare minimum, it's got to be greater than 280 wide for Twitter to accept it.
Again, I very much appreciate your plugin and the time you spend on it.