Drawing a straight line with Pinta

I'm trying to superimpose a straight line onto a .png image. GIMP is notoriously impossible to use for simple things, so people have suggested Pinta . However, as far as I can tell, the only lines you can draw with this program are freeform ones, i.e., there doesn't appear to be anything like a ruler tool. How can I draw a straight line? Or is there another program that does it? Again, I need to be super-imposing on an existing .png file, so Xfig won't work.