One of the enhancements added to Mac OS X with the release of the Leopard operating systems is the Extract Shape tool that has been added to the Preview application. This powerful tool makes it easy for users to cleanly cut items from one photograph and insert them into another with a different background. You can use this to quickly generate neutral backgrounds for your Photo Booth snapshots or to convince your friends for April Fool's Day that you once toured Paris with Pamela Anderson.
Double click the photo in Finder to open it with Preview. If the photo is in iPhoto, then drag it out of iPhoto and into your desktop so you can access it easily with Preview. Don't worry, the original will remain in your iPhoto account, unmodified. This is just a fast way to copy it to a useful place.
Click and hold down your mouse button over "Select" from the menu over the photo. Choose "extract shape" from the drop down menu.
Trace the outline of the area of the image you wish to keep with your left mouse button held down. A red path will follow your mouse pointer, and you want to ensure that the visible edge that seperates your foreground from your background remains within this red area.
Release the mouse button when you have traced the foreground, release your mouse button, the area outside your trace will be faded.
Press enter. The background of the image will disappear and be replaced with grey. However, in reality, the grey is just a filler colour to indicate that the background is transparent. This means you can copy this onto another image and the grey will be replaced with the new background.
Click and drag your mouse over the foreground that you just extracted to create a box around it and release the button. Hit Command C or select "Copy" from the Edit menu.
Open the photo containing the new background you would like to use by double clicking it in Finder.
Press Command V or select "Paste" from the Edit menu. Click and drag your foreground around to decide where you would like it and press enter when you are satisfied with its location.
Extract Shape is a powerful tool, but it isn't fool proof. For good results, the colours of the foreground and background must be very distinct so Preview can properly detect the edge of yoru shape. Removing a white cat from a background of a brown carpet will work well. Removing a man in a black shirt from a night photo will not work so well.