Image mouseover/mouseout attributes and Theme/Skin question

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Hi all,
I am building a web site with theme support, under each theme's folder there
are some images. Some of my images need to be skinned, I know I can build a
skin file with line like

<asp:Image runat="server" ImageUrl="images/email.gif" skinid="Email"
BorderWidth="0" />

Then in my master page or web page I can call this skin like

<asp:Image ID="img" runat="server" SkinID="Email" />

This works perfectly until I need to add mouseout/mouseover client
attributes to this image. Usually what I do is

img.Attributes.Add("onMouseOver", "msover('img','images/email_on.gif');");
img.Attributes.Add("onMouseOut", "msout('img','images/email_off.gif');");

Now, my question is how to pull these on/off images from theme folder?

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Regards
Hardy