ImageSnap is a Public Domain command-line tool that lets you capture still images from an iSight or other video source. You might remember Axel Bauer’s original isightcapture tool (that is no longer supported but still works on most systems). I wanted a similar tool with source available so I could make feature changes or at least recompile with Apple’s ever-changing architectures.
At some point Apple introduced QTKit, a new and oh-so-welcome abstraction bringing QuickTime programming into the 21st century. Nothing against die-hard ANSI C programming, but it fits awkwardly into what are otherwise clean Objective-C Cocoa programs.
Rather than provide a detailed tutorial about using QTKit (sorry if you wanted one), I’ll just point you to the ImageSnap code so you can take a look at a few useful things like starting a QTCaptureSession, capturing output with QTCaptureDecompressedVideoOutput, and saving an NSImage to disk.