BLPConverter.exe converts BLP files (Blizzard's proprietary texture file type) into standard PNG files and back again. This is NOT a UI mod: it is a standalone program. BLPConverter uses a Windows command line interface which allows it to be easily incorporated in batch files/scripts and give the user a great amount of control about the conversion and the created file's format.
BLPConverter's emphasis is on giving the user control over the exact format of the textures it creates since WoW can be quite particular about the format of BLPs it will accept in a given situation. Unfortunately I have only had time to give it a command line interface, which I admit can present a learning curve to people unfamiliar with them. The help file included in the download attempts to minimize this hurdle.
I'm committed to making sure this program can handle all types of BLPs, so if you find any that it can't handle I'd love to hear about it.
Version 8.1 - Added support for BLP_PAL_A4 format.