Watermarking options can be located by going to Admin > Accounts; selecting your account, and finally clicking on Watermark on the left side column.
Now you can set your account's text watermark to repeat across the image. You'll find this under the "Position" option:
Adjust the "Rotation Angle" and "Font Size" to get more or less coverage on the image.
Username Watermark Settings
We broke out the Watermark feature under your Account options, and added a new "Set Username Watermark" feature:
Here, you'll be able to adjust the options for your account's username watermark, which can be used in Galleries. Preview the watermark with your own username: