[Zyonin]

You may want to try converting the PNG to a .tga using a program like The GIMP (which using the standard setting, produces WoW friendly tgas). This what I do when I create custom images and textures for use in game. You file has be mutilples of 16, 32, 64, 128, 256, or 512 or any combo therein such as 32x16

BLPs can be a bit tricker as that is a Blizzard format and can be changed at Blizzard's whim. If you are using an older version, it may not produce WoW friendly BLPs.