Posting Link On Fb And G+

How come when I post a link to my profile page or website it only lets me choose my profile pic or somebody else's photo for the thumbnail? It won't let me choose one of my pictures. It used to let me, but not anymore. I highly doubt my profile pic will inspire anyone to click the link, but I don't want to false advertise and show somebody else's art instead of mine. I hope they fix this.