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.
terrorism
child sexual exploitation and abuse (CSEA) offences, including
grooming
image-based child sexual abuse material (CSAM)
CSAM URLs
hate
harassment, stalking, threats and abuse
controlling or coercive behaviour
intimate image abuse
extreme pornography
sexual exploitation of adults
human trafficking
unlawful immigration
fraud and financial offences
proceeds of crime
drugs and psychoactive substances
firearms, knives and other weapons
encouraging or assisting suicide
foreign interference
animal cruelty