Paste as a Layer - Comparing Images
Main Menu: Edit > Paste as a Layer
Pasting an Image on top of other, as a Layer, is useful to compare two images. A similar thing can be done by using the Merge command.
After an image is pasted as a Layer, either by using the Edit > Paste as a Layer command or by dragging and dropping an image from the Database Pane over the current image, the Paste Tab will appear on top of the image. By using several controls on that Tab it is possibly to set the Opacity, Rotation, Zoom, Horizontal Offset and Vertical Offset for the pasted image.

Adjusting the pasted image in that way you can overlap precisely both images and compare them easily.
This Tab has three buttons:
Clear, which is used to remove the Layer.
Reset, that restores the predefined values of Opacity, Rotation and Offset.
Merge, which merges both the pasted layer and the original image in a single image.
If Auto Reset is checked, any time another image is dropped over, the original values of Opacity, Rotation and Offset will be used, otherwise, the current values will remain in place.