As I said above, if we perform an image reverse search on one of your unsourced images (or anyone else's) and find a similar one online we will treat it as stolen content.
It's the authors responsibility and duty to back up his/hers materials, not ours to hunt and guess where each image comes from.
It's like you driving your car, getting pulled over by a police officer who asks you for your driving license and you reply to him: "I have it but I will not show you because I just don't feel like it".
You might be indeed entitled to drive, but you still have to prove that every time you're asked for.
I'm in Thailand. We do not need licenses here - we bribe :)
But hopefully nothing will show up. I have other places that check for source and attribution and canva posters have never caused a problem. And I would not be using posters for art posts anyway. All of my photos are mine because no one else would take them.