The ImageOverlay replicates icons which are already available and can be derived from the arrow icon and css transforms. Instead of adding new icons for these, we should reuse. To answer Joaquin's question from the comments, the change being asked for will use the same icon but different variations and different orientations. White arrows are already used in the last modified bar.
Benefit: Doing this will reduce the number of bytes the mobile site uses. Icons generally account for quite a bit of css.