We also have a built in control to make it easier to use images from beneath the skin folder in a way that is more portable so that if you copy the skin to a new site with a different site id or change the skin name it still works.
<portal:SkinFolderImage id="img1" runat="server" ImageFileName="MyImages/home_icon.png" />
assuming the folder where you have the image beneath your skin folder is named MyImages