Comentario oficial
This doesn't actually have much to do with your images. That message means that you likely have an image that is not anywhere near the map area included in the dataset.
Look at the dots in the map at the bottom and it will show you to the filename for the errant image.
This could also potentially happen if your Ground Reference image was taken REALLY far away from your mapped area but that is pretty unlikely.
Let's say for example your GSD is 1 inch. 6.4 million pixels being the estimate means that the dataset spans over 100 miles of distance.