webshot不会在html中呈现本地图像

I'm using node-webshot and phantomjs-cli to render an html string to a png image. The html contains an image tag. The image is not rendered when it is the src attribute points to a local file and no error is raised. However it does render the image when the src points to a http location. I've tried all of the different file path combinations that I can think of, eg I'm using node-webshot and phantomjs-cli to ren