I've never been fined in Norway, but procedures are pretty similar in most countries.
First, a photo is not usually needed to make a speeding fine valid. If you think you were not speeding you can contest the fine in court and the police will produce whatever evidence they have that you were speeding. A photo may or may not be included.
When you were given the ticket you should also have been given instructions on how to pay it. Most jurisdictions allow you to pay in person, or online, or by mail, and possibly over the phone by credit card. It's extremely likely that your fine is payable through the Norwegian Government Collections Agency. Details of how to pay through this agency are given in the answer to this question.
In most cases it works that you are fined a certain amount, but that if you pay within a certain time (14 days in your case) they will accept a lower amount. So if you pay after 14 days the fine will be more. However in my experience if you send in the payment by mail before 14 days is up and it arrives a day or two after the 14 days is up they will accept it anyway. Usually but not always.
If you do not pay then the fine remains on your record and you may have trouble if you go back to Norway. What kind of trouble is also covered by the above question.