The decision says that no one can be considered the author of that picture, as copyright only protects human creative works. It is possible that the work infringes someone else's copyright (the decision doesn't address this point at all), but unless that happens, the image is in the public domain and you can freely copy it.
Note that, unlike patents and trademarks, copyright doesn't require registration. The court could have found that, while the AI can't assert copyright, the copyright still exists and belongs to the owner of the AI or the designer of the AI etc. Instead, they found that the work can't be protected by copyright, since it is not the product of human creativity.
Edit: here is a citation from the article that I'm basing the previous assertions on:
> Both in its 2019 decision and its decision this February, the USCO found the “human authorship” element was lacking and was wholly necessary to obtain a copyright, Engadget’s K. Holt wrote. Current copyright law only provides protections to “the fruits of intellectual labor” that “are founded in the creative powers of the [human] mind,” the USCO states.
