No problem, here it is. I use fb open graph and twitter together.
<b:if cond=âdata:post.firstImageUrlâ>
I used this: âdata:post.snippet + âVeja mais no siteââ on description, because I wasnât able to generate a text to image-only posts, or posts that sometimes (donât know why) blogger donât get a snippet. Since the description is needed for summary, I just added a fixed text.