Your file is corrupted for me please upload as tga or better png. I can't convert it back to png.
Make shure you use
BLP2PNG I have tryed all blp converters here and this is still the best one.
When you replace it make shure your file works fine with the castbar AND the arena/BG countdown castbar (has a bad offset and looks silly if you try to make a thin border with the normal castbar)
I photoshoped it the preview to test if your texture is to big for the castbar and added it as blp. Looks a bit ugly new but you can be shure its not about the texture you use but the converting programm.
