The Clone Stamp Tool paints
with a sampled pixels of an image.
In the Toolbox, select the Clone Stamp
Tool.
Choose brush size and style*.
Alt-click (mouse-click holding [Alt] key)
on any opened image to set a sampling point.
Paint.
On the Options bar you can also define the
blending mode, opacity and flow.
The Aligned gadget on the Options
bar (selected by default) forces the starting point to follow
the cursor, even after you complete a stroke (i.e. after you release
the mouse button or uplift the stylus), while leaving it off starts
the sample point back to it's original location at the start of
each stroke.
