I'm just talking about the graphic images in the data folder.
I'll give a specific example:
We all know this image for endure:

I can swap this with some other image like this:

(I just made that in paint in like 10 seconds as an example)
As far as I know, I'd just be changing the graphic that appears when using a skill. Not actually giving me an advantage over other players, only a less bland/standard appearance.
I guess it's the next level of graphic customization.
(IMO a customized client could be, a different skin, a custom aura, custom buff images, custom words appearing when soullinking/asura striking, custom wiz AoE spells)
I guess that I can get ready to start opening an art shop for this then ^^
Since it is just a "cosmetic change"