So I can see this being a nice simple time killer, if you don't know the trick to tic-tac-toe (as in you should win at best and tie at worst).
The AI is clearly thought out when it starts and the randomization to avoid a permanent tie situation is well done.
The graphics could have used more attention instead of what appears to be just quick images made in paint. I too use paint for my graphics, so you can do some nicer things.
Also, if you have the talent or know someone who does, a little background music never hurts.
All in all keep up the work, don't get discouraged and keep on getting better!