Facebook recently has made a change to shared link thumbnail image size. The whole layout of the share has been changed as well. The big image is now located under the text, not in the left column.
While this new layout yet to be transitioned to the regular API shares, there is a way to make this happen right now. Version 3 has an option to let Facebook decide the share info by itself using your OG tags and other info.
If you check this option and your image is up to the specifications Facebook will make your share presented in the new layout. The recommended og:image size for this feature is 1200x627px, minimum is 560x292px.
Update: Version 3.0 has been released.
Update 2 (January 2014): New layout is available to all types of Facebook shares including API shares.
If you don’t see the big thumbnail then your link doesn’t meet the Facebook requirements for big thumbnails.
There are several:
– link MUST be shared, not attached.
– page must have og: meta tags.
– og:image meta tag must be set and contain the correct image URL.
– size of the image listed in og:image meta tag should be minimum is 560x292px, recommended 1200x627px.
Live example – Embeded autoshare of this exact post: