To remove the default sorting placeholders (the double arrow image) from the headers, go to the style folder that you are using (e.g. Grid/styles/black_glass) and perform these two operations:
- delete (or rename) the "placeholder.gif" image;
- rename "placeholder_empty.gif" to "placeholder.gif".
"placeholder_empty.gif" is a transparent image having the same size as the default "placeholder.gif".
By renaming it to "placeholder.gif" it will be used by the grid instead of the double arrow image.