Yes you can! You can also edit your profile image and your name (which is your watermark) at any time. To edit any of these fields, just click the "edit profile" button on your profile page, make any changes you wish and then click ‘Update your profile’.
A little heads-up here: changing your name will update all the watermarks on all your images. This process can take a minute or two so you may see a temporary message displayed on your image pages whilst this is updating in the background!