ns_gifsize - Determine the width and height of a GIF87 or GIF89 image
Returns a list of two elements, the width and height of the GIF file specified by filename. An error is generated if the file does not exist or if it is not a GIF file.
% ns_gifsize screenshot.gif 1024 768
% ns_gifsize test.html invalid GIF file: "test.html"
ns_imgmime, ns_imgsize, ns_imgtype, ns_jpegsize, ns_pngsize
global built-in, images