I am confused about Property releases for art. If you want to upload photo of a piece of art you should fill in Property release form. In most of these release The Property owner gives the Photographer and the agency some rights...
But who is the Property owner - The author or the person that is the owner of the physical peace of art?

As Steve said, owning the artwork doesn't make you the copyrightholder. And with replacing artwork with my own photos in an image I'm uploading (which I've done) I've typically had to upload a property release for that "art" as agencies will reject, assuming the images belong to someone else.

"Art" is pretty broad - I use my own release for some handpainted easter eggs that I made as props too. Agencies are getting much more careful about property releases.

Thank you for your answers. Generally I agree but every time when I write PR I have the feeling that I do something that is not necessary.

In my case I paint watercolors on paper and after scan/photo I upload them in different agencies. Writing different PR for every illustration for every agency is annoying.

I am always asking myself - Why when I make vector illustration I just upload it and when make watercolor illustration on paper I have to fill in PR. In both cases I am the author and it is my obligation to upload only my work - i agreed with it when I signed the agencies terms and conditions.
And also in all PR forms I give the photographer and the agency some rights ... What if there is no photographer - there is a scanner. It is a machine. And if I do not give any rights to the photographer because he does not exist why isn't it enough my agreement with the agency to sell  my work(with all rights and limitations in this agreement).