Transparent GIFs
A transparent GIF file will allow you to make ONE color disappear.
This becomes important when designing your own buttons for a theme.
Notice these buttons on the left, the space between these buttons is
caused by transparent GIFs.
Note: Only GIF files can have a transparent
color, JPG files can not. GIF files are limited to 256 colors. JPG
files can be 16.7 million colors.

Examples