Do not assume all of your images are the same size. The Logical Screen Block also chooses one of the colors in the Global Color Table to be the Background color of the screen. This color selection is ignored by Netscape Navigator. This will allows background GIFs to fill in the logical screen background. A GIF file contains a global palette of common colors for all the images in its file to work from.
Palettes are very important.
Every color displayed in your GIF must come from a palette. The fewer colors used, the easier it will be for systems to display your images. The global palette is applied to all images in a GIF file. However, no image can every reference more than one palette, so 256 colors per image is the max. Having a bunch of local palettes with wildly varied colors can sometimes cause color shifts in your display.