Della, click on your profile at the top of the page, then click on "Change your site preferences". A list of "Preferences" comes up, one of them being "Add a watermark to uploaded photos?" The drop down menu will give you choices to use for a watermark which will be added to all photos you upload to the site.
Take a look at the other preferences offered and adjust to suit your taste. Don't forget to click on "Save your changes" when you are done making your choices.