signature backgroundImage doesn't accept an absolute path

Steps to Reproduce

        <signature id="sig" width="600px" height="300px"
                   backgroundImage="/bg.jpg" 
                   backgroundIncluded="true"/>
        <image id="image2" src="/bg.jpg"/>

1. put bg.jpg under the root folder

Current Result

signature doesn't show the background image
but the image component can show bg.jpg as expected

Expected Result

signature can show the background image

Debug Information

signature doesn't encode image URL like the image src attribute