I downloaded IrfanView from:

http://www.irfanview.com/

The name is a misnomer; it is an editor, not just a viewer. It lets you crop & resize any photo format, and it also has effects: you can blur out faces, etc.

You can also print words on the photos (your name or handle, for example). It was pretty easy to figure out.

It won't do everything Adobe Photoshop or Gimp will do, but it's easier to use.

I like this viewer although I use another program (Paint Shot Pro) for serious editing. Good porn should be viewed without any distractions such as window borders or tool bars. Irfanview lets you blow the picture up to be a full screen. I especially like being able to zoom towards the best parts of the pictures.