I think you are misunderstanding the property. All properties that end with CssClass are css class names. This property ends in Image, which means it is a reference to an Image src. (I believe you found this out already).
Looking at the code a ResolveUrl is called. From the MSDN docs.
If the relativeUrl parameter contains an absolute URL, the URL is returned unchanged. If the relativeUrl parameter contains a relative URL, that URL is changed to a relative URL that is correct for the current request path, so that the browser can resolve the URL.
For example, consider the following scenario:
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A client has requested an ASP.NET page that contains a user control that has an image associated with it.
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The ASP.NET page is located at /Store/page1.aspx.
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The user control is located at /Store/UserControls/UC1.ascx.
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The image file is located at /UserControls/Images/Image1.jpg.
If the user control passes the relative path to the image (that is, /Store/UserControls/Images/Image1.jpg) to the ResolveUrl method, the method will return the value /Images/Image1.jpg.
So it looks like if you pass in an absolute url you would be ok... the question for the skinner is whether there is a replacement token that can be used that will allow the xml property to be prefixed with the skin path...