The white halo effect your seeing is a result of anti-aliasing. As GIFs don't support any opacities other than 0 and 100 percent, the somewhat transparent parts of the image are given a level of the anti-aliasing color so it looks like it's blending into what background color you want. If the anti-aliasing color is the same color as the background you want the image to sit on, the halo will not be visible.