One might expect that if you only installed a nulled version to "try it out" you wouldn't have many users or much content to lose...
However, once you purchase XenForo properly, download the files from your customer area for the version you are currently using and then upload them, replacing any existing files on your server. After that I would recommend going to the install page and clicking the "Rebuild Master Data" button.
If you are currently using an older version of XenForo, you may also want to consider upgrading. We released XF 1.5.6 today.
If using a nulled.. I'd be tempted to delete ALL the files (including all in the /data and /internal_data) due to the fact you don't know what was put into the PHP files that may have opened a back door. Of course, that's the hazard of using nulled software - they frequently come with a back door installed.
Otherwise, I'd be running numerous checks via scanners to make sure there were no root-kits installed and try to ascertain if the images have had code inserted into them.
You running on a VPS/dedi or a shared hosting environment? In shared hosting, it's going to be more difficult.... on a VPS/dedi there are utilities that will help scan, but none are fool-proof.
chkrootkit is handy for root kits.
The biggest issue is that you don't know WHAT code (if any) was in the nulled version... so you don't know what/where/if you opened a back door into the system itself that allowed remote access to others - and if so, what they may have done/replaced.
Personally.. I'd wipe the VPS totally and start from scratch - but I'm paranoid about that kind of stuff.