Tutorial in case some don't know
DO TAKE NOTE:
FAQ #332: Does deviantART have a feature that will automatically watermark my deviations?
"This option will be presented to you only if you change the Display Options field to one of the smaller resolutions available in the submission/editing process, allowing you to add a watermark or not.
Images which are kept at 'original size' will not be processed by us and will not present the option of automatic watermarking on your images."
Also, if you submit your image under certain categories, or your image type (jpeg, png etc) is different from the usual, the watermark option will not appear.