note that in PHP 4.2.2 and PDF_lib 4.0.1 this function does not support more than 256 colors (only GD 1.x it seems).
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);