From a technical point of view stuff that's embedded in the file format of the file can be stripped out real easily with almost any kind of paint package and you can get rid of invisible watermarks real easily too. Usually a stretch of the image gets rid of most but you can go further with some more specialist stuff if need be.
If you looking for what to do that's hard to get rid of then putting a logo in a place on the map that's not so easy to just crop out and fill in with background is the best. Use a logo that lots of detail and lots of colors. If you want to go to the extreme then use a logo made up with a random dot fill in it. Oh and don't blend the logo in and then reuse exactly the same one in lots of images. You can ask the computer to find the similarities between lots of images and extract the logo. If the random dot fill is changing each time then you cant use that method.
Its not that I have had to do this you understand but removing a feint watermark from images is no different to removing constant background noise from astro images which I have done quite a lot of.
Well its not like its supposed to be uncrackable, its just to detter most people and those without technical ability, and anyway theres no perfect safeguard, plus then theres, plus theres the date of creation of originals plus deveopment files if it has to be legally proved i guess