You can use the Palette Manager to designate a transparent color in an image. Making a color transparent is helpful if you want to see information that is displayed behind the image. The transparent color is also used to replace the data that is removed when you edit an image using the rub or crop commands. If background transparency is also turned on for the image, the rubbed areas (or the slack space in a cropped image) become transparent.
As with most changes in the Palette Manager dialog box, the changes you make to the palette are not permanent until you click the Apply button.
For more information, see Transparent Color of an Image.