Much of things boils down to doing a risk assessment and deciding on mitigations.
Unfortunately we live in a world where if you allow users to upload and share images, with zero checks, you are disturbingly likely to end up hosting CSAM.
Ofcom have guides, risk assessment tools and more, if you think any of this is relevant to you that's a good place to start.
https://www.ofcom.org.uk/online-safety/illegal-and-harmful-c...
If I ran a small forum in the UK I would shut it down - not worth risk of jail time for getting it wrong.