<asp:Image runat="server" id="LogoImage" SkinId="LogoImage" />

public void Page_PreInit(object sender, EventArgs e)
{
  LogoImage.SkinID = "LogoImage";
}

var path = "~/App_Themes/" + Page.Theme + "/images";
var path = "~/App_Themes/" + Page.StylesheetTheme + "/images";

<asp:Image runat="server" id="myImage" ImageUrl='<%# GetThemedImage(((Product)Container.DataItem).Image)%>' />

public string GetThemedImage(string image)
{
     return "~/App_Themes/" + Page.Theme + "/images/" + image;
}