niki,
what software do you have for post processing....... once we know that will be better placed to advise.
Some people on here, have a pretty good watermark.... one that springs to mind ~(in my opinion) is Colin ("Hacker") - with his wedding photos.
A lot of people just use a little bit of white text - which with the Content Aware filter in Cs6 Beta, you can get rid of in 2 seconds! - i.e. you might as well not bother....
Bit of a trade off on logo visibility v photo quality.. i.e. at times, a logo can dominate the photo too much....