No, it's not a bug. VaultWiki doesn't currently offer any specific preview-image information to Facebook or other sites. If sometimes an image appears and sometimes it doesn't, then it has to do with the algorithms that Facebook uses to "find" a preview-image. We don't know what those are exactly, but it might be hindered in your second example because the image is wrapped in B tags. An image isn't text, so it can't be bold. The B tag might cause Facebook to skip its contents (the picture) because it thinks it's not possible for a picture to be inside it.