If you want to open a TIFF image, you have to use pdf_open_image_file(). If you want to open a TIFF image from memory, you'll have to wait until version 5. I gleaned this from the PDFlib folks themselves:
> from rp@pdflib.com
The only way to open TIFF with PDFlib 4 is to open it from
file. But in PDFlib 5 (http://www.pdflib.com/beta/) we add
a virtual file system, so you can easy open a file from
memory.