Thanks for letting us know. With a quick look at the
documentation here
in the "Pre-caching images" section, FB now mentions that the two ways to get image pre-cached is:
1. Pre-cache the image with the Sharing Debugger
2. Use og:image:width and og:image:height Open Graph tags
I don't remember seeing this before in the docs, so it looks like adding those tags should do the trick going forward.
-Melissa