Should I be concerned about someone removing a watermark I add to my image? Are there steps I can take to ensure that it's not possible?
Can someone remove the watermark from your image? Yes, of course.
Is it easy? No, not without the right tools
Just like with any other area of human endeavor, it's an arms race. Whatever protection can be devised can be circumvented. Whatever attack is known, can be defended against. It's a never-ending cycle.
A watermark on an image keeps honest people honest, just like a lock on a door. But a watermark (or any other security scheme) will never protect from someone with true larcenous intent.
A good question to ask is: How much value do I place on this image, and how much time and money do I want to spend to make it a little harder (not impossible) for someone to use without my consent?
One method to consider is that instead of adding a water mark, is to add more of a stamp im some place that it won't be easily removable. Such as, if you have a picture of a person, put a black (c) partly over their face. This will force someone to actually recreate part of the image vs just removing a watermark. But of course, this looks pretty ugly. It would depend on how you're using the watermarked image of course.
Probably the best method is to only make a small version of your image available. People can copy it, but it will just look plain ugly, which won't stop people, but it's better than nothing :)