Someone who's in charge of updating and maintaining an online site or page is a webmaster. If you want to submit a photo of your dog to IncrediblyAdorableAnimals.com, you should email the webmaster.
These days, the term webmaster isn't quite as common as it once was — you might see job titles like "website administrator" or "web developer" used instead. Still, it's not unusual for smaller websites to have one person who's in charge of many different aspects of keeping the site functioning and looking good, from design to analyzing traffic to troubleshooting problems. Since this person is in charge of the website, it's appropriate to refer to her as a webmaster.