It looks like you took Bob's advice - which is spot on. Check the "internet headers" that are a "hidden" part of every email. What email client do you use? I'd be happy to help get specific steps for you to see the Internet Headers for your mail client.
There is no need to think that you are a zombie computer and sending pictures, emails, spam and malware across the net. Make sure you have an anti-virus program, run one or two of the spyware packages and make sure the PC is clean.
This happens to most, if not all, of us. there are many ways spammers can guess or harvest your email address and spoof the sender info.
Again - check the Internet Headers - they'll show you that the mail didn't originate from your machine or really come from you.