Using the clone tool

The "clone" or "rubber stamp" tool let's you copy pixels from one part of an image to another.

Here's a .jpg scan of an old photo. On the surface of the original photo are some significant smudges. We'll use the rubber stamp tool to correct these.

1. Select the rubber-stamp tool.
2. Select an appropriate brush size.
3. Alt-click on a non-smudged part of the image that is the same color/texture as you'd like the smudged area to be,
4. Next move the mouse to the smudged part of the image, hold down the mouse button, and start to "paint" over the smudge.

Sometimes you can hide the fact that you're cloning by using a speckled brush, and cloning in from several different positions in the image, rather than just painting from one part.