How to define an icon for my context menu item?

How can I define an icon for a context menu item without declaring it in manifest.json?

For example I want to offer the possibility to change the icon in the options.

I thought I could use something like that, but nothing found in the Docs:

chrome.contextMenus.create( {'type': 'normal',
                             'title': 'CustomMenu',
          开发者_StackOverflow中文版                   'image': 'http://www.example.org/img.png'
                            }
                           );

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From looking at the current documentation: http://code.google.com/chrome/extensions/contextMenus.html You cannot update the icon.