For viewing images (and PDF files, at the very least -- not sure what all it can do), the Preview app which comes with OS X is plenty decent.
If you want to get on to doing some basic image editing (something not nearly as substantial as CS3), check out Seashore.
Screenshots? There are at least 3 different ways: Full screen (with drop-shadow), region (no drop-shadow) and window (with drop-shadow).