Watermarks are all different. Some are plastered all over the photo, some are diagonal from top to bottom. Some are small and along the bottom, though this means they can be cropped.
Ones I really like belong to The Pink Sith. I love the name of the blog and I really love the logo. They are quirky, original and instantly recognizable. Plus you cannot crop them out or Photoshop them away without going to more trouble than its worth. My guess is anyone browsing Google images would scroll right past these to find easier prey.
Here are two examples of how she uses them. She is brilliant. She is really well known now and I have heard that even Disney, who owns the Star Wars franchise, is a fan.