How to take a picture with the mac webcam in PNG?
The version of imagesnap distributed through Homebrew (and available on https://github.com/rharder/imagesnap) doesn't support PNG. Apparently support for other formats besides JPG was removed when QTKit got replaced by AVFoundation.

If you download 0.2.5 directly from http://iharder.sourceforge.net/current/macosx/imagesnap/ you get a version which also supports other formats.

$ ./imagesnap -h
USAGE: ./imagesnap [options] [filename]
Version: 0.2.5
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
  -t x.xx     Take a picture every x.xx seconds
  -q          Quiet mode. Do not output any text
  -w x.xx     Warmup. Delay snapshot x.xx seconds after turning on camera
  -d device   Use named video device