There is always a debate on watermarking your images, with some saying it ruins images and others insisting that it is an important step to protecting your work online.
If you are happy to post your images online without a watermark then that's your choice, as much as it's others choice to place a watermark.

Lets start with the basic principle:
A watermark is a semi-transparent marking on an image, created to protect the image itself from being credited to another person or becoming an Orphan Work.



What should a watermark contain?
This is entirely up to you - you can include a copyright statement ( © 2012 Richard Horsfield ), or a website address (www.richardhorsfield.com) or anything else you want.

How big should it be?
Given that the watermark is there to protect the image, it's generally not a good idea to put your watermark in a corner, as this can then be easily cropped out.
As the watermark should be semi-transparent it can be placed anywhere on the image, and should not detract from the image too much.
A big, bold copyright symbol in the middle of the image is not necessarily the best option.

Some photographers and Agencies create a repeated watermark that contains small text which covers the whole image.

It's entirely your choice how you watermark your work, have a look at images online from other photographers and see what they use. If you find something you like you can try to adapt it to suit you.
Also remember that it may be necessary to have more than one watermarking option to suit various online needs.

Logos
Remember that if you have a logo, this should be placed on the image at 100% opacity. Your logo is your branding and should be treated separately to watermarking.