With Preview in Leopard, you can now combine multiple images into one image. Open Image #1, click and drag to make a selection, copy to the clipboard. Then open Image #2 and paste the clipboard contents. From there, you can drag the selection to reposition it.You can also use Preview's new selection tools (Extract Shape and Instant Alpha) to make an irregular shaped selection. When you have part of an image selected, you can also Command-Drag the corner handles to rotate the selection. Here's an example of the results that can be achieved with minimal effort.

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