There are two different ways to access the set transparency menu. One is through the color menu: Click color and select Set Palette Transparency. It will look like this:
The second way is to access the set transparency menu is to put a set transparency button on the task bar. Go to file/preferences/customize toolbar. This is what you will see.
Click on Set Transparency, then click add and it will pop onto the taskbar and in the menu in the second box. Then click ok.
Once you have finished your graphic flood fill the background of your graphic with a color that is not in the graphic, (or else this color has also been there the whole time since you used it in the beginning.) Use the eyedropper to sample the background color, and right click to make it the background color. Or if the background color is the "top" color, click on the black arrow to make it go to the back.
Now select the transparency menu in whatever way works well for you. The next three boxes will pop up in succession:
Now, your gif should be transparent. If you want to check, you can go to the color menu and select View Transparency. Your gif should look like this:
When you post your graphic on your website, it should look like this:
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