Facebook is testing a new feature that adds a litte bit more context to your notifications.
Some users are now seeing thumbnail photos within their notifications, when that notification has to do with a photo. For instance, if you were tagged in a photo, or if someone commented on a photoed that you're tagged in, you may see the new thumbnail next to the notification.
It's a feature that's a long time coming, as it allows users to read the photo comment in context without having to go to the actual photo. Check it out:
AllFacebook obtained this screencap that shows the photo thumbnail previews appearing on the admin panel for page admins as well:
We're seeing this new thumbnails, are you?