Now let's create a background that will be transparent so that a wallpaper background of a web page will show through.
Even though the background is transparent now, if we make it a Gif image now, the layers will flatten and the checkerboard areas will turn white.
We could choose those white areas to be transparent during the export process, but that might be a problem if we had some white areas in the cartoon, like perhaps the pages of the book or if our cartoon was a person or animal with white teeth.
Then those white areas would become transparent too.

So, let's go back to the layers palette and create a new, empty layer.
Name this layer something like 'bkgrnd' and drag this layer to the bottom of the layer stack.

Now we need to pick a color that will be transparent in our final Gif89a cartoon. We need a color that is not in the body of our cartoon and a color that will not leave a "halo" of odd colored pixels around the outside of it either.

To best accomplish this, choose a light colored background if your cartoon will go on a light colored wallpaper.
Try to choose a color that will be close to the color of the wallpaper if you can.

[Note: For web cartoons, use a color palette that is browser-friendly. Your images will load faster and be more consistently colored across the most popular browsers if you do.
You can download one here.
The cursor below is a bucket, though it should show an eyedropper. Unfortunately, as I press SHIFT/OPTION/3 to take this screen shot, it automatically changes to a bucket. But you get the idea.]

Now fill the empty 'bkgrnd' layer with the color you chose in the step above.

Then if you are satisfied with how everything looks, open 'Image' from the title bar selections and choose 'Mode'.

Change the mode of your cartoon to 'Indexed Color'.

When it asks if you want to flatten layers, choose OK.

Then Export your cartoon to Gif89a format.

During the Gif89a export process, a dialog box will appear where you can use the eyedropper tool to pick a color to be transparent.
Pick your light-colored background and click OK.