You can use pretty much any license you want — but make sure the image really is CC0.

There are a wide array of Creative Commons licenses meant to cover different situations. CC0 is special — it is a "No Rights Reserved" declaration. It exists because not all jurisdictions have a clear way to dedicate something to the public domain, even if you want to. This gives a consistent legal framework to do so.

From the FAQ:

Can anyone use a work that is distributed under CC0?

Yes. CC0 doesn’t restrict who can use a CC0’d work. Once applied, anyone can use the work in any way and for any purpose, including commercial purposes, subject to rights others may have in the work or how it’s used, as well as subject to any other laws or restrictions that may apply.

Do I have to attribute the person who applied CC0 to their work?

No, there is no legal requirement that you attribute the affirmer, only an expectation that you will voluntarily do so if requested.

Only CC licenses with the SA clause (like CC-BY-SA 3.0 under which this site is licensed), require creators of a derivative work to license their contributions under the same terms. If the artwork you want to use is under CC-BY, for example, you need to provide clear attribution and indicate what changes you made, but you don't need to license the combined work in any particular way.

CC0 goes to the extreme of giving things up. There's no SA, and there's no BY. You can basically do whatever you want. (Read the legal text for details — or better, if you're concerned, have your lawyer do it, because things which appear to be plain English in legal documents often have deeper specific legal meaning — it's like a code.)

But, one note: if you're doing anything serious, and particularly, if you're doing something commercial, I really, reallly reallllly suggest checking that the original author is the one who has chosen this license. There are many image / wallpaper / "free download!" site where people upload images of dubious origin and claim CC0 when they do not necessarily have the right to do so.

For example, since you mention Pixabay.... I found this lovely image of a tiger, which claims to be under CC0. However, a reverse image search takes me right to this image on Shutterstock. Shutterstock's terms are royalty free, but decidedly not CC0 ­— see Shutterstock license comparison.

I don't even know for sure that the Shutterstock image is the original one (although it seems much more likely). In the case of the particular images you are interested in, the cloud image is also found on Shutterstock, but appears to have been put there by the same person as on Pixabay.

In any case, be careful not to perpetuate this. It's definitely not safe to just pick things that say CC0 on Pixabay, unfortunately.