Negative value for graphicXOffset not being applied Please compare these two maps.

On this map ...
http://dev.lambeth.gotpantheon.com/map
Notice the location police helmet image, bottom-left near to Union Road.

On this map ...
http://test.lambeth.gotpantheon.com/map
The actual location of this police station is indicated by a blue circle.

The problem is that we need to have the graphic raised by 20px or so so that it points to the correct location.

When we add a negative graphicXOffset in the style settings, nothing happens. (A positive value does work, but that's not what we want).

See screenshots here: http://drupal.org/node/1808794
Actually, I've successfully used both negative x and y offsets.

However, since you're trying to raise the graphic (move it up, I'm assuming), you need to be using the y offset.  That's the one that controls the vertical placement.  ;0)