When you share a post on Facebook, or when Jetpack Publicize publishes a post to your Facebook page, Facebook crawls the page and looks for Open Graph meta tags in the head to build a complete post preview (with an image, title, description, …).
As you discovered, Facebook has a debug tool to let you know what they see when they crawl your site. The “og:image could not be downloaded or is too small” error is usually due to the featured image or uploaded image not meeting Facebook’s size requirements, which are:
1. At least 200×200 pixels in both width and height
2. Aspect ratio of width and height no greater than 3; in other words, the width can’t be more than three times more than the height, and vice versa.
If both of those aren’t met, Facebook will crawl your site to find an image that does meet those requirements and they randomly pick whatever image they find.
However, the image being shown in your post “Vascoites go Mountaineering!” post is 6.2mb in size and is most likely not be served up fast enough for Facebook when your posts are scraped for content – which seems to be a known bug on Facebook’s end:
https://developers.facebook.com/bugs/294523810715538/
I would recommend uploading smaller sizes of your photos for the blog so they can be loaded more quickly so they don’t time out for Facebook’s servers until they address this bug. If you check your other posts that didn’t work, the images might also be too big.
I hope that helps!