You can use the BLP to PNG converter.
http://www.wowinterface.com/download...Converter.html
Important. Your texture width/height needs to be a multiplier of 8.
16,32,64,128,256,512,1024 are possible values for width and height. The texture does not have to be square.
Not sure if a single texture can use 2048 in width/height yet. Try to save your texture in 2048x1024. If it loads OK. Otherwise I would try two 1024x1024 textures.