Sure. Just follow these steps below. These are likely steps you've already followed, but it bears stating for others who may run into the same issue:

1. make sure the author of the work has explicitly stated which license their work is being shared under. (If the author said "use for whatever, I don't care" that is not a license. You must get explicit permission to share it under one of our licenses or the author must have already explicitly listed one of our accepted licenses for the work.)
2. make sure the work has not already been shared here, by the original author or someone else sharing it on their behalf, just as you are planning to do. (Search the site for the art you plan to share. If it doesn't show up, we'd love to have it!)
3. make sure the work is useful to others, not just one or two highly niche projects. (easy: most assets are useful for more than one thing. This is really just to prevent people from uploading stuff like gameplay screenshots as advertisements or pictures of text)
4. Upload the art, select the license, indicate the original author- as well as a URL to the original asset- in the Copyright/Attribution Notice section. (Attribution is not strictly required for CC0, but its still a good idea and everyone is encouraged to give attribution anyway since it helps us prove the author did indeed give permission and/or licensed it CC0 if anyone becomes unsure if it safe to use.)
5. Mark the asset "Is this your own work? YES". (It isn't (entirely) your own work, but that component of the site is currently broken, and the fact that it isn't yours is made clear by the attribution and URL. The intent is functionality, not deception :) I'm trying to get it fixed, but it is considered low priority, unfortunately.)