Check for presence of image

Image will return, even when `nil`.
```
#<KnowledgeBase::ImageUploader:0x007fd28c524a48
 @model=
  #<KnowledgeBase::Sectionables::Text id: 1, heading: "laudantiumnullaeum", lead: "Nihil laudantium sunt consequatur rem ipsam sit non...", body: "Accusantium natus asperiores totam aut porro et. Ni...", created_at: "2014-03-25 09:27:42", updated_at: "2014-03-25 09:27:42", image: nil>,
 @mounted_as=:image>
```