In this photo we are seeing an ill-defined wall cloud.  How can you tell it's a wall cloud and not just scud?  There's a few subtle hints.  Wall clouds form at the base of an updraft, so often times spotters will observe quick upward motion within the wall cloud itself.  While this is not possible in this still image, notice that the right-hand side of the wall cloud angles down a bit lower than the rest of the feature.  This is because the updraft is ingesting rain-cooled air from the right-hand side of the photo.  This cool air condenses and forms the low hanging wall cloud.  Also, an observer should pay close attention to the small ragged "fingers" hanging from the wall cloud to see if they are rotating.  If they are, a funnel cloud or tornado may develop soon.