As reported at http://www.php.net/manual/en/function.pdf-open-memory-image.php
ImageCreateTrueColor and others does not work correctly when using
pdflib > 4.0.x and PHP > 4.2.x and reading the image from memory (reading from
file seems to work though).

Excerpt from forum and configuration information below.

WORKS:
$im = ImageCreate(100, 100);
$pim = pdf_open_memory_image($pdf, $im);

DOES NOT WORK:
$im = ImageCreateTrueColor(100, 100);
$pim = pdf_open_memory_image($pdf, $im);

DOES NOT WORK:
$im = ImageCreateFromJPEG("file.jpg");
$pim = pdf_open_memory_image($pdf, $im);

-----------------------
PHP 4.2.3
GD 2.0.8
PDFLib 4.0.3 (compiled with default options)

Configure Options:

./configure \
                      --with-config-file-path=/etc \
                      --with-apxs=/usr/local/apache/bin/apxs \
                      --enable-versioning \
                      --enable-inline-optimization \
                      --enable-magic-quotes \
                      --enable-track-vars \
                      --with-xml --with-dom --with-xmlrpc \
                      --with-mysql=/usr \
                      --enable-sockets \
                      --with-ldap \
                      --with-imap=/usr/local --with-imap-ssl=/usr/local \
                      --with-mm \
                      --enable-memory-limit \
                      --enable-bcmath \
                      --enable-sigchild \
                      --with-zlib=/usr \
                      --with-curl=/usr/local \
                      --with-dom=/usr/include/libxml2/libxml \
                      --enable-exif \
                      --enable-ftp \
                      --with-gettext=/usr/share \
                      --with-gd \
                      --with-ttf \
                      --with-freetype-dir=/usr/local/include/freetype2/freetype \
                      --enable-gd-imgstrttf \
                      --with-png-dir=/usr \
                      --with-jpeg-dir=/usr \
                      --with-tiff-lib=/usr \
                      --with-mcal=../libmcal \
                      --with-openssl \
                      --with-mcrypt --with-mhash \
                      --with-pspell \
                      --with-ming \
                      --with-pdflib