What free simple software is best for editing photos - Paint won't do what I want! - details below?

I have a photo (from outside Buckingham Palace yesterday!) that I want to rotate only SLIGHTLY as it's slightly wonky. I just want to straighten it up. Paint will only rotate by a minimum of 90 degrees apparently. Can anyone suggest something very simple I can do? THANK YOU - 10 points for best and simplest answer that works!