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A few of the configuration options changes (and some were lost) in VS2010 (as part of the rework in WPF).
You can do some customisation in the Commands tab of the customised dialogue (that you get to by right clicking on the toolbar). But controlling the image is not one of them.
Command Image Changing Extension
http://blogs.msdn.com/b/visualstudio/archive/2010/06/17/command-image-changing-extension.aspx