Halo Effect

(Graphics) The glow that appears around an antialiased image when it is placed over a background color that is different from the background used when making it antialiased. This occurs for gif images, which allow transparency, but only for an individual pixel to be either 100% transparent or not, not for instance, 50% transparent. Thus, when transparent, antialiased gif images need to be created, they need to be created on a background that is the same color or a near color to their final destination background color.