Transparency is only available in indexed colors mode, so to define a
transparency color, follow these steps :
• Select the menu item "Image / Mode / Indexed color"
• Convert the image to and indexed color format by choosing a suitable
number of colors from the list.
• Select the menu item "Image / Transparency color"
• While the "Transparent color" window is open, select the color to be
transparent by clicking on it in the image by using the pipette (the cursor
changes to the pipette automatically while doing this)