Only 'form associated' elements have a potential form owner.

Source/core/html/HTMLImageElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 63 matching lines @@
 
     bool hasPendingActivity() const { return m_imageLoader.hasPendingActivity(); }
 
     virtual bool canContainRangeEndPoint() const { return false; }
 
     void addClient(ImageLoaderClient* client) { m_imageLoader.addClient(client); }
     void removeClient(ImageLoaderClient* client) { m_imageLoader.removeClient(client); }
 
     virtual const AtomicString imageSourceURL() const OVERRIDE;
 
+    virtual HTMLFormElement* formOwner() const OVERRIDE;
+
 protected:
     explicit HTMLImageElement(Document&, HTMLFormElement* = 0);
 
     virtual void didMoveToNewDocument(Document& oldDocument) OVERRIDE;
 
 private:
     virtual bool areAuthorShadowsAllowed() const OVERRIDE { return false; }
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
     virtual bool isPresentationAttribute(const QualifiedName&) const OVERRIDE;
@@ skipping 22 matching lines @@
     CompositeOperator m_compositeOperator;
     AtomicString m_bestFitImageURL;
     float m_imageDevicePixelRatio;
 };
 
 DEFINE_NODE_TYPE_CASTS(HTMLImageElement, hasTagName(HTMLNames::imgTag));
 
 } //namespace
 
 #endif