If imagecreatefromjpeg() fails with "PHP Fatal error: Call to undefined function: imagecreatefromjpeg()", it does NOT necessarily mean that you don't have GD installed.
If phpinfo() shows GD, but not JPEG support, then that's the problem. You would think that --with-gd would do the right thing since it does check for the existance of libjpeg (and finds it) and add that feature to GD, but it doesn't in v4.4.4 at least on RHEL v2.1, RHEL v3, CentOS v2.1 or CentOS v4.3.
On those platforms, it's *important* that --with-jpeg-dir be *before* --with-gd. If it's not, GD won't build with jpeg support as if --with-jpeg-dir had never been specified...
