That behavior would occur when the quad is not set to autosize to the size of the PNG. If the quad is larger than the PNG then it will use the "Mode" setting in the Texture Coords tab to determine what to do to fill the extra space. The default is "Clamp" which will repeat the edge pixels. If you choose "Border" it will add a transparent border to fill the extra space.
So, to fix, either enable "Autosizing" or set Texture Coord Mode to Border.
#XPression