If the texture you used is the same that the one you decided to show us, it's normal that it doesn't work : the picture has a 274x267 format with a white part on it, so it can't be read. You must only use, as Filipsons said, powers of two : so you can only use values like 64, 124, 254, 512, etc...
You can't just cut a texture picture like that, if you move everything by only one pixel, it can change dramatically the look of the whole model by making it awful ! Especialy for creatures.
Here, it is what it looks like when you put all the white stuff off. This one should work. I let you convert it. The next time, you'll have to do it by your own.
If you want to have a better quality texture, you must make it with a 512x512 format. (Or 1024x1024... But let's not go too far please...)
About Bolvar's texture, you can export it from WowModelViewer, but this is a special kind of texture, so the texture you'll have will not be the true one... You must export the blp and open it with BLPlaboratory, and then you can take take the texture. You'll have this :
But if you don't a burning man, you'll need to change the picture. For instance if you want it to be purple like in your picture, it's easy to make :
