Do images need to be copyrighted before uploading?
If you own the images, you don't need to copyright them. In fact watermarks are NOT allowed on submissions.
However, if you don't own the image rights or the subjects depicted in them, you must submit a signed property release by the actual owner/artist before you can sell them on Adobe Stock.
Also see model releases.
If you took the picture, you own the copyright. Job done. No need to register. (You need to own ALL the copyright, for example a photo containing someone else's photo, the McDonald's sign, branded shoes, a car, or a building may have shared copyright with the original owners, and you can't make money from it).