This is not an advert – just a suggestion to social-photo uploaders and some pros!.
They say: Marksta allows you to watermark your photography, adding text or a logo directly from your iPhone or iPad. Add a copyright notice to your images before sharing them to Facebook, Twitter, Instagram, Tumblr, Flickr, etc. Or add your Twitter name, or your website address. Whatever you want to write, Marksta makes it simple and fast.
Cost: £1.49 Marksta.com
There are a few other apps on the market but a few pro photographers have written in saying they are using it. Simple!
Update: this comment in from photographer George Solomonides
Just tested out the free version of Marksta. Very simple to use. Mentioned that it came through PhotoAtchiveNews on Facebook.