Hello hello!
The way you do it is, you do the challenge and post it on your tumblr blog (they're free and easy to setup!), then you go to the Sexy Art Challenges site's Submit page.
Choose Link from the pull-down menu that usually says Photo. Paste the URL in of the specific post from your tumblr. Then I'll reblog it on the official tumblr, and you can reblog it too, if you like!
You can feel free to share a link in this topic, but please do be sure to warn people if there's any mature content or stuff that may be considered not "work-safe". We don't want to break any of Tapastic's rules, after all!
Hope this helps clear things up. 