I think GIMP is overkill here, as I am not one to use a bazooka as a fly-swatter. As for IrfanView, I found it to be a hassle; I don’t like the interface and have a few other issues with it.
My vote is XnView , another free program that allows you to converts, resize, crop, adjust the colors/brightness, and do anything short of spot touch-ups without the overkill factor of GIMP or the irksomeness of IrfanView. It’s worked for me for years, and I’ve even done some restoratin work with it; something I could never do with IrfanView.
Better yet, try them both and see which one you like better.