Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 145

Collecting Image Information and Adding It to the Image
The first step is to select the point from which the image information is to be
copied so you can transfer it to the blemish and correct it. Point the mouse
cursor at an apt spot near a blemish. Press the Ctrl key and hold it down. The
cursor will take the shape of a crosshair. Click the left mouse button on a non-
blemished area that resembles the blemish while holding the Ctrl key down.
Figure 2.67
The image dustandscratches.png before the touchup
Then, first release the mouse button and then the Ctrl key.
When you left-click on the blemish, the image information you just
copied will be placed there. Point to another blemish and repeat the process.
Since you have selected the Aligned option, the point from which the image
information was taken will move with your Clone Tool. Continue working until
you need to copy new image data to correct blemishes in different areas.
Repeat the process (i.e., select a new point from which to copy information,
press and hold down the Ctrl key, left-click, release the mouse button and Ctrl
key, and then “stamp” out the blemish by left-clicking on it).
Changing Brush and View
The selected brush, 85 px, is ideal for removing spots along the wall and in the
flower beds. If you want to remove the scratch or dog-ear in the upper-right
corner, you should select a brush with a smaller pointer (say, 45 px) from the
Brushes window. Or you can simply scale the brush in the tools options.