This is the configstring of the server running potato r3 and php 4.0.5:

'./configure' '--with-mysql' '--with-apache=../apache_1.3.19' '--enable-track-vars' '--enable-trans-sid' '--with-mcrypt=../libmcrypt-2.4.10' '--disable-posix-threads' '--with-gd=/usr/local' '--with-jpeg-dir=/usr/local' '--with-ttf=/usr/local/include/freetype2' '--enable-shared' '--enable-gd-imgstrttf' '--enable-versioning' '--enable-magic-quotes'

As GetImageSize didn't work on a specific JPEG image I tried the same image on another server running Debian GNU/Linux Woody and php 4.0.4pl1 with the following configstring:

'./configure' '--with-apache=../apache_1.3.19' '--with-unixODBC=/usr/local' '--with-mysql=/usr/local/mysql' '--enable-track-vars' '--enable-gd-imgstrttf' '--with-gd=/usr/local' '--with-jpeg-dir=/usr/local' '--with-ttf=/usr/include/freetype' '--enable-trans-sid' '--with-mcrypt' '--enable-debug'

And on that server the GetImageSize() function just returned the image dimensions as you would suspect..

ImageMagick's identify returns the following information on the image:
Image: sicko.jpg
  format: JPEG (Joint Photographic Experts Group JFIF format)
  type: true color
  class: DirectClass
  matte: False
  colors: 117318
  geometry: 1024x768
  depth: 8
  filesize: 98kb
  interlace: None
  background-color: gray74
  border-color: gray74
  matte-color: gray74
  compression: JPEG
  runlength packets: 639759 of 786432
  tainted: False
  signature: 96e8d70ecf6d1784d27f801c39ec7bc5