It doesn't have to be explicit; it's not a forced content, but it is a freedom to do things that are more explicit than are allowed here, if you so choose.
I'm asking that everyone make a tumblr, but they're free and easy to make, and easy to post on. It will just be easier for me to organize and, since the Tapastic forums are still not fixed, anything we crosspost here would have to be uploaded somewhere else anyway for most people.
The rules and guidelines, as well as instructions, are on the official site's FAQ, so just check that out and hopefully it'll answer all your questions!
I'll be making a topic here for every prompt, although of course every piece of art will probably not be possible to post here as well as on tumblr, since Tapastic has less permissive terms than tumblr. But we can still post some of the pieces (probably!) or point people to tumblr to check them out, if they so choose. My posts about the challenges here will mostly serve as reminders to people wishing to participate.
Just let me know if you have any other questions! 
