This is all very helpful, thank you :)

Any Creative Commons license will require the festival to list a credit for your work. That might not be an issue for the app or website, but the festival might not be interested in listing “poster by Tippon, used under a Creative Commons 4.0 license” every time they use something you created on social media, in print, or advertising.

I didn't know about that. I thought they might have to credit me, but didn't know that it would need to be on the work itself. I definitely don't want that. I hadn't thought about third parties either. I doubt that anyone would try to rip us off, but I don't want to leave that door open either, just in case.

I'm not overly concerned with future clients, as I'm medically retired and just doing this as a learning exercise. There's a tiny chance that I might get future work from this, but I'm more concerned with not cutting myself off from using the website / app template if I was to fall out with the festival at some point. As you say though, I don't want to leave either me or the festival open to having the content stolen either.

I've just realised that I need to double check how to handle third party images too. We have some local businesses who support the festival, so we use their logos as links on the site. If I include them in the repository, I don't want to accidentally give them away.

I've got some reading to do! :D