The reason that there arent watermarks and whatnot to check the integrity of 'original' space images is that there is no such thing. The instruments on our shuttles, probes, and astronaut's 'cameras' do not just snap an image on to some film or even something as simple as your digital camera. Lots and lots of data is captured by instruments observing space, and it takes months of work to collaborate all the data into a context that creates an image depicting what the scene would have looked like to human eyes. We're talking teraquads of data. Just ask Phil Plait, he goes into this well in his BadAstronomy website.

Another reason there aren't security key's, and Phil goes over this too (but not for this reason) is that it would go against the spirit of the NASA version of an open information act, which declares that after one year, all the data from a particular project on Hubble (or whatever instrument) is open business to the public. Watermarking 'original' images would be serious trouble.