Use Flood Erase 
to fill areas of the picture to make them transparent. The pointer changes to the Flood Erase pointer 
. The area to be filled is defined by other colors surrounding it or the edges of the work area. Click to fill an area.
Flood Erase will not flow across gaps between separate areas of the same color. Either fill each area separately, or use other drawing tools to 'patch up' a shape to make it a continuous area of the same color.
settings to select the color variation for the area to be erased. See Tolerance.
Shortcut
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Paint Window Control Panel; Tools:
Remove a Background with Flood Erase
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Click the Flood Erase icon
and the pointer changes to the Flood Erase pointer
. -
Change the color sensitivity of Flood Erase with Tolerance
, by adjusting the slider.
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Click on a color within your picture. Adjacent areas in the picture with a similar color to the area that you clicked are erased.