I use Photoshop to place a subtle translucent watermark across the whole image diagonally.
As has been stated, watermarks in the corners can be cropped out by the less scrupulous individuals.
My watermarks are transparent but still clearly show who's work they are without detracting too much from the image quality/subject matter.
With Photoshop, you can adjust the level of 'opacity' quite easily if you think its not strong enough.
