After selecting a group of images to merge, the following dialog box will come up:
A list of the selected images will appear. If you have defined an image overlap while importing, It is recommended that you select the Z ->A ordering before merging to retain that overlap.
By default the "Remove original images from drawing" will be selected, in which case only the final merged image will remain in the project. You can deselect this option if you would also like to keep the original images.