A meme that started from 4chan and comes from Suiseiseki of Rozen Maiden, and her tendancy of saying "desu" after everything. The starter of the meme is when /a/ spammed /b/ with it, and they adopted it as a meme. It is usually used as spam.
The tag should only be used if "desu" or "です" appears in the image, or in rare cases where the character Suiseiseki is used as a replacement for the term (post #266173).