I noticed that, if I specify relative URL path for an image, it prepends "http://xxx.xx.xxx/" (Ex:http://120.121.35.134/) to get the image path. It so happens that sometimes this is not correct as the site may go through various proxies and PHP might not always return the correct path. In this case, where it can't find the image, mpdf displays no_image.jpg just fine.
How can I make sure I supply image paths just like no_image.jpg instead of specifying relative or absolute paths that have dificulties resolving themselves?
If you supply relative paths and PHP can access that with functions like fopen() then mPDF will use the relative path supplied. If you have enabled safe mode, or open_basedir in your PHP this may affect access.
You can get no_image for other reasons. Some image types require temporary writing of data to the /tmp/ folder which must have write permissions.