Picasa is no longer supported however there is no reason why one can't still use it - it's just an image viewer, not a browser and while threats for practically everything exist, I doubt you'd come across one anytime soon. My brother favors this one and he only had good things to say about it.
Picasa is really good imo. If you ever take a look, in the settings you can add pic extensions to associate with Picasa and also locations. It's true it's only mild editing, but there are some interesting tools there. If this were still supported I wonder if Google would have run all the pic hosting sites out of business. It was really ahead of its time.
I used FastStone for some time. It was nice. It was light for me on editing tools too, so I went back to using Irfanview/Paint.net/Inkscape to deal with graphics and pic viewing. I guess I would like something with slightly better editing than Irfanview for viewing, but it's OK for me.