Re: PDF generation?
If an Anonymous person cannot view the image, then the image will not be in the PDF generated file. Without logging in, does the link http://mysite.com/show_image.php?id=20 show the image? If not. then that is the problem. You may have to put the image into an image gallery that has anonymous read access.
For tw to create a PDF file, it processes the contents of the wiki page and generates the appropriate PDF syntax. This translation in 1.9.x is performed with the PDFlib library (in the lib/pdflib folder) For images, function img() in pdflib.php is where images are opened and processed.
In 1.10, the translation is performed by the fpdf library http://fpdf.org . I have had more success with 1.10 PDF generation than with 1.9.
Gary