If you're dealing with a work that didn't include a notification, and you know what the correct notification should be (say, a friend took a picture and said you could post it, but you want to make sure others know it belongs to your friend) you could add the copyright symbol and their name as a courtesy. But you could not add the copyright symbol and your own name without negotiating a transfer of copyright.
The copyright holder; unless other arrangements have been made, this is likely to be the creator.
Unless other arrangements are made, the creator of the work is the automatic copyright holder.
Unless other arrangements were made, the creator is the initial copyright holder.
Unless other arrangements were made, the creator is automatically the copyright holder.
Unless other arrangements are made, the creator of a work controls the copyright.
Unless other arrangements were made, the creator would be the copyright holder.
No; the creator retains the copyright unless other arrangements are made.
Unless other agreements were made, the creator of a work is the copyright holder. However, copyright is transferable.
Unless other arrangements were made, the creator is the initial copyright holder.
Unless other agreements have been made, the creator is considered the copyright holder.
Unless other arrangements were made, the creator controls the copyright.
The creator is the first rightsholder, unless other arrangements were made.