content: If someone has root (administrator) access on your machine assume that absolutely anything is possible.  They can download anything, erase anything, change logs, etc. Depending on how sophisticated the software is, how much time has elapsed since the porn was downloaded, and how competent the forensic team is that's analyzing the logs, it may be possible to prove your innocence, but not very likely. If someone ever did plant that kind of stuff on your computer, and then reported you to the police, your only hope would be that the attack was clumsy enough to leave evidence.