I too have wondered this. As the majority of views are coming from mobile devices, we are advised to go for a full width (good quality) image. That said it could depend on your subject. If your subject is one which may get 'pinned' on Pinterest then you should opt for a vertical (portrait) image.
If your topic is better suited to Twitter, opt for a landscape (horizontal) image.
There are some writers here on Hubpages who opt for no image and prefer to have an ad placed there.
Above may keep your viewers interested. Remember from a sales point of view, items above the fold, as Isaac Asante mentioned, get noticed.