about an online watermark tool earlier today. There are several, like picmarkr.com
; Google "online watermark tool" or something similar to find some more.
But I would recommend, like Mel1206 said, familiarizing yourself with graphic or photo software to do it yourself for better customization. I use Photoshop currently to do it. Not necessarily because I think it looks nice but mostly because I go nuts seeing one of my images pinned to pinterest and it's sourced to Google or someone who's stolen the graphic and not my blog