[OpenLayers-Dev] why are there 2+ img locations?

[Dejung Gewissler]

Hi Devs-

In trying to better understand OpenLayers and how to customize aspects 
of it I came across three different locations for images.
These are:
    - openlayers\img
    - openlayers\theme\default\img
    - openlayers\art

Does anyone have any insight as to why they live separately?
In choosing where to add my own customized images for the panZoom 
controls or other toolbars (like editing) I am ambiguous and need help :)

Thanks,
Dejung