I use facebook share button for every post in my blog.
(http://loveclubhouse.blogspot.com/)
When there was no Open Graph meta tag included in my blog, facebook would retrieve the first picture from the post's page to make a preview story on the sharer's wall for this post.
Once, I added an Open Graph image meta tag to my blog:
<meta expr:content='data:blog.postImageThumbnailUrl' property='og:image'/>
However, the size of postImageThumbnails turned out to be too small, so I decided not to use them anymore and removed the above metnioned Open Graph meta tag from my blog.
The problem is:
Even though I removed all Open Graph meta tags from my blog, facebook still somehow creates preview stories for my posts based on those tiny data:blog.postImageThumbnailUrl and doesn't want anymore to retrieve the first pictures from posts content as it was before I made any changes in my blog.