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 91 matching lines @@
 
     virtual bool draggable() const;
 
     virtual void addSubresourceAttributeURLs(ListHashSet<KURL>&) const;
 
     virtual InsertionNotificationRequest insertedInto(ContainerNode*) OVERRIDE;
     virtual void removedFrom(ContainerNode*) OVERRIDE;
     virtual bool shouldRegisterAsNamedItem() const OVERRIDE { return true; }
     virtual bool shouldRegisterAsExtraNamedItem() const OVERRIDE { return true; }
     virtual bool isInteractiveContent() const OVERRIDE;
+    virtual bool isFormAssociatedElement() const OVERRIDE { return true; }
     virtual Image* imageContents() OVERRIDE;
 
     HTMLImageLoader m_imageLoader;
     HTMLFormElement* m_form;
     CompositeOperator m_compositeOperator;
     AtomicString m_bestFitImageURL;
     float m_imageDevicePixelRatio;
 };
 
 DEFINE_NODE_TYPE_CASTS(HTMLImageElement, hasTagName(HTMLNames::imgTag));
 
 } //namespace
 
 #endif