#2 and #4 are underexposed. You can't fix that in post. Except to a certain extent if you shoot raw.
#3 is underexposed in some areas and overexposed in others. This would be a good case for some
conservative use of HDR. Key word being conservative.
Great compositions though!
My advice would be to learn Photoshop, GIMP, or Aperture (Photoshop being the easiest and probably best, imo). Picasa is extremely limited. It's worth the time invested to learn better software. There's plenty of tutorials on YouTube.