Not any visible difference, yet WoW only supports .blp (Blizzard's propriety format) and .tga graphic files. .png is just an intermediate step useful for people designing graphics.
Personally, I always use .tga because of Windows' file structure. While there are extensions to preview .blp in Windows, .tga and .png are displayed by default. This allows me to skip the whole .png step and GIMP/PS don't support .blp so there's that too.
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