I was in the US 2 years on J2, 3 months on F2 and 2 years and a half on F1. Am I a resident for tax purposes?

That would dependent what years these visits in te US cover.
If you were in the US constantly form January 2008 for example, you would still be a non-resident on F1 for 2012.
Could you provide more details?

If you got off the pl,an in 2007 or earlier, you were a resident alien for tax purposes for 2012. If 2008 was your arrival year, you become a resident alien THIS year.