And don't forget 2x2.gif, with the top left and bottom right pixel transparent, and the other two pixels coloured white or grey. Used to make background images

And don't forget 2x2.gif, with the top left and bottom right pixel transparent, and the other two pixels coloured white or grey. Used to make background images of table cells with this, and that would create a faux opacity effect with whatever was used as the background image behind the table.