ID:273195
 
I've been fiddling with pixel_x and pixel_y of the overlay for awhile and it keeps going on the sides.
With isometric projection, each tile has width, length and height (whereas in top-down there wasn't height, and length was used to simulate it). To displace an atom's icon by height, they added the pixel_z var (which you would've found yourself if you had briefly looked up the other pixel vars in the Ref!).
Altering the pixel_x or pixel_y values could change which tile the overlay effectively belongs to. If you want the overlay to appear directly above the mob, use the new pixel_z value.

Lummox JR
In response to Kaioken
I did mess with pixel_z too, but it appeared to have only put an overlay -under- a turf, as it did not show on one, but on no turf, it would.

EDIT: I had forgot, I made the walls be a layer above most turfs. pixel_z=32 puts an overlay above you, if anyone is to do a forum search and stumbles upon this.