Here is a photo of the monument to the American Navy at Utah Beach in Normandy, France.

We can use the Instant Alpha tool to replace the peaceful blue sky and instead put in a real documentary photo of American forces landing at Utah Beach on D-day in 1944.

The Alpha tool removes background in photos. Apply Instant Alpha from under the Format menu. When Alpha is active, the cursor turns into a little cross and a dark panel with instructions for using Alpha appears in the document. Drag the cross over the background to rub it out, several times if necessary, and then hit Return. The sky in this photo has light blue and white colors, contrasting with the dark texture of the monument. This is why Alpha easily picks out and deletes the background. It is more difficult to apply to photos where the background has colors similar to the ones of the main figure. Be careful not to rub out chunks of the object that you want to keep in the picture.

Alignment Guides will help two photos to "snap" together. Position the second, background photo against the monument, so that the dark figures in it are set apart from the central sculpture of the Navy commander. The black-and-white photo can be masked, and the mask is set to have the same width as the main photo.

Now the composition gives a symbolic link between the present day and history.

If you had a wedding photo or a snapshot of a beach party on a day when the sky was gray, you could "improve" it by replacing it with a picture of the sky on a bright sunny day.

The same technique can be applied to many other tasks, such as:

In recent versions of Mac OS X, Alpha is included as a tool in Preview. You can prepare images without their background, separately, for use across all applications on your Mac.