We've used a bunch of different things, but Eyebatch is far and away the best I've run into. It's like $30 or something and allows flexibility in location of the watermark, automatic/batch resizing, multiple steps (as in, open pic | watermark | save | resize to thumbnail | save again | rename | save again )

It also supports sequential numbering and renaming, format conversion (.bmp to .jpg or whatever in about 15 different formats). And no, I don't have any connection with the company that makes it