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

Source/core/html/HTMLImageElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 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 96 matching lines @@
         addPropertyToPresentationAttributeStyle(style, CSSPropertyVerticalAlign, value);
     else
         HTMLElement::collectStyleForPresentationAttribute(name, value, style);
 }
 
 const AtomicString HTMLImageElement::imageSourceURL() const
 {
     return m_bestFitImageURL.isNull() ? fastGetAttribute(srcAttr) : m_bestFitImageURL;
 }
 
+HTMLFormElement* HTMLImageElement::formOwner() const
+{
+    return findFormAncestor();
+}
+
 void HTMLImageElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
     if (name == altAttr) {
         if (renderer() && renderer()->isImage())
             toRenderImage(renderer())->updateAltText();
     } else if (name == srcAttr || name == srcsetAttr) {
         if (RuntimeEnabledFeatures::srcsetEnabled()) {
             ImageCandidate candidate = bestFitSourceForImageAttributes(document().devicePixelRatio(), fastGetAttribute(srcAttr), fastGetAttribute(srcsetAttr));
             m_bestFitImageURL = candidate.toString();
             float candidateScaleFactor = candidate.scaleFactor();
@@ skipping 262 matching lines @@
 
     return m_imageLoader.image()->image();
 }
 
 bool HTMLImageElement::isInteractiveContent() const
 {
     return fastHasAttribute(usemapAttr);
 }
 
 }