bullets should be centered and additional bullet graphics

I noticed that the bullet's top left corner is placed where the mouse click happened. It should rather get centered above the x/y locations.
In the 12x12 pix blue bullet example it should be moved 6px up and to the left. I don't know enough about css to see if this could be done in the css.

If you use larger bullet graphics it gets more important to get them centered correctly.

I attached some different bullets that could be used, just change the location in line 31 of imagemarker.css and change the width and height to 36px. Don't forget to clear the cache.