Open the image you wish to insert. Select the Magic Wand Tool and click into the empty space of the imported image. The empty area around the image will be highlighted.
Now click on Select, then click on Inverse. The image you want to copy is now highlighted.
Click on Edit, then click on Copy (this will copy the image into the clipboard)
Now, click into your new image to make it the active window. Click on Edit, click on Paste and your image will be pasted into your new document.