App for hi-res frame capture from a webcam
How about imagesnap. If it's simple you want, then that's a free (with source) command line tool with the following options:

USAGE: ./imagesnap [options] [filename]
Version: 0.2.4
Captures an image from a video device and saves it in a file.
If no device is specified, the system default will be used.
If no filename is specfied, snapshot.jpg will be used.
Supported image types: JPEG, TIFF, PNG, GIF, BMP
  -h          This help message
  -v          Verbose mode
  -l          List available video devices
  -d device   Use named video device


I've just tried it on my 10.7.3 Macbook Pro (early 2011) and it captures images at 1280 x 720 from the built-in camera (which I beleive is its full resolution).