Use the Paintbrush 
to paint lines on the picture that follow the movement of the pointer. Choose one of the Brush Shapes, and set its Size and Angle. See Brush Types.
Shortcut
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Paint Window Control Panel, Tools:
Draw a Line with the Paintbrush
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Select the desired color in the Color panel. Use Pick Color
if you want to select a color from the picture.
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Click the Paintbrush icon
.
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Change the line width and style if desired by selecting a Brush Shape, Size and Angle.
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Click and drag across the work area from the desired start point to the desired end point to draw a line. A line appears showing the freehand line as it is drawn. Lift the pen or release the mouse button and the line is completed.
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Each time you drag and release a new line will be drawn, until another feature is selected.
You can single-click to place dots of color.