This new feature, like automatic camera uploads, is opt-in and can also be turned off at any time.
From the Dropbox Blog:
Starting today, all the screenshots you take can automatically be saved straight to your Dropbox. And on top of that, Dropbox will also create a link to your screenshot and copy it to your clipboard — so your picture’s instantly good to share.
As an added bonus for all you Mac users, today’s update also includes a nice little importer that copies your photos from iPhoto directly to your Dropbox.