Set this attribute to have the treecol element use an image for the header instead of a label. You cannot have both an image and a label displayed in the header. Set this to the URL of an image to appear on the tree column header. If this attribute is left out, no image appears and the label is used instead. The class treecol-image must be used on the treecol element for the image to appear. You should still specify a label for use in the column picker if ignoreincolumnpicker is not true and hidecolumnpicker is not true on the tree.