This property is used to set and get the the source for the image to be displayed
in the image box. The source can be set with either an image file path string or an
stream. If the source is a image file path, then this property returns that
string. If source was set as a stream, then this property returns a null.
var imgBox = scene.getBoxById("startImgBox");
if (imgBox)
{
imgBox.src = "box://app/resources/testPicture.png";
}