Mtk,
I'm a bit confused about this myself. I just did a bit of testing with different images. What I found was that whenever 2 images were set, the 'image is too small' text would appear in the debugger. There are multiple bug reports in Facebook regarding the og:image property and the error your seeing:
developers.facebook.com/bugs/338739036234971
Facebook definitely changed things lately regarding how that's parsed, and I think that's the problem. One of the big problems people mention is if your og:image tag redirects from the URL you specify to the actual image. That isn't an issue on your site, or on our test sites though.
So, unfortunately, my only suggestions are to:
* Try removing your "default" image in the Open Graph settings area and see if that message goes away. For us it did.
* Use the bug report above and use the "Does this repro describe your issue?". Describe the problem like you did here and wait for FB to fix the issue.
Hope that helps, and thanks for the report. We'll definitely be monitoring it.
Alex