Tolerance:
How close the selected transparent color has to be to the color you wish to be transparent. 0 means it must be the exact color while a larger number means a larger range of color difference will be accepted.
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Opacity:
How opaque the image will be. A value of 100 will make the image completely opaque (fully visible). A value of 0 will make the image completely transparent (invisible). Values in between can be used to make the image translucent (see-through).