Don't do it!
I hired an online personal trainer about a year or so ago. Yes, he sent me routines to do, but I was basically on my own. Sometimes, even with the animated pictures, I did not understand how to do the exercises he was giving me. No one was there to correct my form.
At first he was very responsive to emails, but after a while he became difficult to contact. Sometimes I would log into his site and the next workout routine would not be posted in time.
Eventually I could not get ahold of him at all! I had paid for 3 months and this was only about halfway through that. I finally received an email from him saying something like "Sorry I've been in the process of moving." I paid this man 300 dollars and was not getting the service I should have.
After that email I replied and was ready to finish out the training but again he never responded. I never heard from him again after that one email.
I'm not saying all of them are crappy like this guy, but even during the good first few weeks I realized an online trainer was NOT what I needed. I need someone who is there with me to make sure I'm exercising correctly. I need someone there to push me towards my limits more because I'm too easy on myself.
I just need someone there. Period.
This year I hired an actual trainer at my gym and have been very happy about it. I see him once a week and it makes a huge difference!