One of the handiest cross-application functions on any computer is the clipboard. It allows you to cut, copy or paste data from one file, such as a cropped image, into another file. Although the clipboard works with only one piece of data at a time, it is still useful for moving cropped imaged from one photo to the other. Preview, the standard image viewing software for all modern Macs, allows you to copy and paste one image onto another.
Double-click on the image file you want to crop to open it in Preview. Click on the picture and drag the cross hairs over the part of the photo you want to keep. Once you have the image cropped to your liking select "Copy" from the Edit menu.
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Select "Open" from Preview's File menu and navigate to the photo onto which you want to paste the first photo. Click "Open."
Click on the second image and select "Paste" from the Edit menu. The cropped image will appear on the open photo. Move the cropped image to the place you want it, then select "Save" from the File menu or press "Command-S" on the keyboard.