I am having the same problem using dw2pdf. I have two wikis though, and notice that one works and one does not. If I serve the wiki from a domain name the images are inserted into the generated pdfs. But when I server the wiki from a localhost on my windows machine, at
http://localhost:8801/... the images are not inserted into the pdfs. Monitoring the http requests (using Httpfox within Firefox) I see that when mpdf requests the images it looks for them at
http://localhost/... rather than at
http://localhost:8801/... It seems that mpdf doesn't include the port number, and consequently can not find the images.
But I don't know how to get it to use the entire address, including the port number.
(Sorry, I don't speak/read French but I saw your post and it seemed relevant. I deciphered it only by using a dictionary!)