In my opinion, MS paint: easy, free, well enough.
But, not everyone has my opinion, so I would recommend Paint tool Sai or Photoshop CS. I don't expect you to be able to get PSCS, but you could be a rich bastard and able to afford it, unlike people like me who eat out of the garbage, so that could be a good choice.
Sai, though, is cheaper and just as effective, thought it misses some tools PSCS has, but those aren't necessary.
There are others and it really depends on the person using it.
