Who owns the photos I upload? I need to be sure about this for my contracts.

You do — the terms of service are explicit: you retain full ownership of your content. What Snaptide gets is a license to process, host and display your photos so the service can actually function — uploading, indexing faces, delivering to guests. That's the standard arrangement for any hosting platform, and it's stated in plain terms rather than buried. If you leave, your photos leave with you; the platform isn't claiming your work.