I am not happy that people can copy my images if they want to, but that is unfortunately part of the price of doing business online. The fact is, many programs exist that can remove a watermark, even from the middle of an image. What I do is put my shop name in the corner so in the off chance someone wants to use the image, they might leave that on.
If you do put a watermark across the middle of your image, it makes it hard to see what it looks like, and that will turn off many buyers. I personally might have bought a print or two if the watermark wasn't so bad it wasn't exactly clear what the whole image looked like.
Further, you won't get into treasuries with large watermarks if that matters to you. I upload all my images with the minimum 72 dpi and usually no larger than 8" on the longest side at that resolution. Yes, it's true some images might be stolen, but the chances of that affecting your sales are low, and it seems the cons of watermarks outweigh the pros.