Picasa is a very comprehensive paint program as well as a picture
viewer, and it has some limited filter features. FastStone, on the
other hand, is a picture viewer also having some more robust filtering
capabilities.
To be fully competitive with Picasa, I suggest there is no better than
PhotoFiltre, with original paint capability, robust editing features,
and a great assortment of filter capabilities (plus many extra plug-in
amenities, such as making thumbnails, etc.) PhotoFiltre comes closest
IMHO to the upper line of commercial paint programs.