Choosing New Spell Icons?

[whak]

I'm dissatisfied with the number of options available for the in-game macro icons, I'd like to be able to more easily differentiate between spells that are mapped to the same spell icon. I'm able to edit and use my own .dds files at this point (so I thought this would be more of an xml question than a dds question), but can someone point me in the right direction as to how I would change where a certain spell would grab its icon from? Let me know if you need any other info...thanks in advance.