First off, Creative Commons is not a government system. It's just a licensing scheme established by a nonprofit to try and encourage some creators to share their works publicly with standardized limitations. It's no guarantee you won't run into trouble. And I don't see anywhere in what they describe this "don't claim ownership" limitation for their CC0 category.
You think you downloaded some "public domain" image? Really? Where? How sure are you it's public domain? Why are you even asking about CC0 licenses? A lot of the time people think they've found "public domain" images, and they have not. Everything is copyright protected automatically unless the creator clearly indicates otherwise. And it's not uncommon to find person B taking something from person A, then trying to say person A's work is public domain... when it's not. So if you got something from person B saying it's "public domain," it could be utterly worthless. Are you sure it was person B's work?
As far as "claiming ownership," you've made a derivative work. When it comes to copyright protection of derivative works based on public domain works, you'd have protection only for the original elements of your own work. For example, if you're Disney and you do your version of Alice in Wonderland, you have protection only for your own original elements, not for the whole thing.
In your sample, adjusting the perspective isn't really doing anything original. Filling in a colored sky may be moderately original, depending on where you got it from and how you did it, but that's still not the core value of the image. And because so much of your image is based on work supposedly public domain, you'd have a lot of problems trying to protect your own work.
Ignoring the Creative Commons issue, if you were to take somebody else's copyrighted work, you'd have to engage in "transformative use" -- with a totally new original message -- for the new work to be "yours." A derivative work is not "yours." And unless you're a professional artist like Andy Warhol doing soup cans, you're not likely to meet the transformative use standard. And even if you do meet the transformative use standard, you're not likely to have the time, energy, or resources to defend it in court if necessary, nor find those kinds of battles worthwhile.