Now that Picnik and Flickr are integrated, you can easily add text watermarks to Flickr pictures.
To watermark a image on Flickr, click the “Edit Photo” link to open that photograph inside Picnik.
Go to Create -> Text and type the text. Now select any font for the watermark from the drop-down and then drag the Fade slider to around 55%. That will make your text slightly transparent so the image portions beneath it are also visible.
To add multiple bookmarks, just do a copy-paste and arrange the text boxes anywhere on the image.
When you are done, Picnik lets you overwrite the original photo with the watermarked image or you can add it as a new picture leaving the old one untouched.
Adding watermarks is destructive editing so you could keep you public photos watermarked while keep a private copy of your non-watermarked images.