You could try
TextureAtlasViewer
If you want to convert from/to .blp/.png then
BLPNGConverter
and/or
BLP2PNG
There's also
wow.tools but it's not being maintained these days so...
There also the game (if you know an element through something like
/fstack you can often find the associated texture in the code files).
I'm sure there's others.
The game also accepts .png files these days so you're not limited to just .tga or .blp if you're creating your own.