You can submit all your photos under a pseudonym by changing the name on your Profile http://www.geograph.org.uk/profile.php (link top write on most pages). Note that by doing this, the name will be changed on all photos you have previously submitted from the account. These may already have been used elsewhere, crediting the name originally shown.
You can change the credit on an individual image, for instance if you asked someone else to take it for you, but the name on your profile will still be shown on the photo page and the photographer name will still link back to your profile.
You can open another account under a pseudonym but this will need to be done from a different email address and you will have to take care which account you are signed in with before submitting, making changes or posting in the forums.