Thanks, I imagined so already. I have tried with .jnx too, same problem: all Birdseye files will be loaded at the same time.
I'll try the suggested software and find out if my .jpg or .kmz files can be converted into .img files....
In the meantime I am already using multiple SD cards, of which I have plenty - but it is a bit troublesome to keep changing those small things in the field.