To add a few details:
I'm aware that fictional depictions of illegal activity are not themselves illegal. but I've no interest in dealing with accusations of promoting illegal activity or obscenity. No adult/child fantasies, period. No non-consensual sex, except within the bounds of consensual BDSM. Etc.
I prefer stories that are tasteful. If you just post lists of explicit sex acts, I probably won't read it. If you post anything scatalogical, I'll delete it. Be sure you keep copies of your own work, because I don't promise to leave it on the site.
Keep copies of your own work in any case. The last StoryBoard died a painful death at the hands of spammers and my non-involvement. This may do the same.
As for explicit images - I'd say that tasteful nudes that you can legally share, with consent and permission would be okay. Keep unsolicited dick pics to the messaging apps where they (don't) belong. Or any other depictions of genitalia.