You could get busted, no doubt about that. It is unlikely though, unless you post a lot of content they are unlikely to even notice you. Plus there are a lot of big usenet posters that ain't got into any sort of trouble yet. A few have had their accounts disabled or posting privileges taken away but that is all.
Just so you know, when you post something to usenet it's not anonymous. Pretty much all servers will either add your IP address to the post body or add an X-Trace line. X-Trace is an encrypted line that only the NSP can decrypt, will contain either the account username or IP address that posted the file.