evidently what you think is white isn't exactly white on your images. It should not render the color if it is set to transparent. Open the image in a program that you can view the color value of a pixel (such as photoshop) and verify that red,green, and blue values are all 255 (&Hff). If they aren't(and i am POSITIVE this is the issue) then you have two choices: use a floodfill in a graphics program to replace the off-white with white in every single image,
or two: compute the correct value based on those three values (using the rgb function) and enter it as the transparency color in the listview. This is obviously much easier than the other method.