Videos have priority over pictures, so if you create a Blogger post with a graphic/picture above a video the video will still show up in Google plus and in your most popular posts not the graphic/picture.
If you do not have a video in a post than Google will show the graphic.
If you use Google Plus and not that many people do compared to Facebook, it is frustrating how Google does this. There should be a way for a Blogger to select which video and or graphic/picture to use, instead of Google just deciding via their algorithms.
The worst part is if a Blogger post has a graphic/picture Google plus will use the video from your "Featured Post" instead of what is on the actual post.
Please see screen shot below to see exactly what I mean.