No problem, here it is. I use fb open graph and twitter together.
<b:if cond=‘data:post.firstImageUrl’>
I used this: ‘data:post.snippet + “Veja mais no site”’ on description, because I wasn’t able to generate a text to image-only posts, or posts that sometimes (don’t know why) blogger don’t get a snippet. Since the description is needed for summary, I just added a fixed text.