Ok, I'm posting this for anyone else who might want to change any textures, and is having trouble getting them to appear. When you enter the folder containing the textures you wish to replace, make sure the textures and subfolder containing said textures are properly named.

For some reason or another, the files for the prestige armors are named royal instead, so you'll have the filename "RoyalCataphractHelmet' or whatever.
This causes the game to use the default textures since there is no "royal" armor set, only the prestige variant.

To fix this and to get your edited textures to show in game, just change the name of any misnamed file to the correct name'variant type. For example, if you wanted to insert your own prestige helmet texture you'd change the name from "RoyalCataphrfactHelmet" to "PrestigeCataphractHelmet".

As for the actual editing, open up the desired PNG in your transparency capable image editing program and go nuts.