Sharing Pinterest Pins on Twitter, sometimes image is included in tweet, sometimes not?

Problem: I am on Pinterest. I select an image to pin to my boards. While pinning, I am offered the option to post to twitter, which I select.

Sometimes the tweet that shows up on Twitter includes the image that I pinned, other times it is only the text description of the image with a link back to Pinterest.

Question: Any idea how to tell which images will post to twitter, and which will result in only a text tweet?

Any help appreciated.