Switch to the Target Window after loading and alignment of the two source images. Select various options, found in the target menu to blend the two images together. Adjust brightness and contrast if necessary. There is another option to apply final tweaks to the size, position, scaling and rotation of a source image.

You can save the complete image to file or alternatively select a rectangular portion and save that to a file.

Check out the align window explanation and you will see the edge of the mona image. Using the fade_in option in the merge dialog you are able to remove these edges as shown below.