Dear Giancarlo,
I don't think I understand precisely your case - I'm still experimenting with an uprade of our production site...
I think you need to verify that all the quizzes and assignments have been upgraded. See the upgrade tools in the site adminsitration panel.
Then perhaps work your way through the quizzes, or have your teachers do so, and ensure that they work, i.e. that the images are there. etc
If you want to recover the original image filenames - the names may be in the XML files or in the question edits. Otherwise I think your only hope are database searches IF you know the new file strings.
To extract jpeg files from a backup (and get all the **new** file strings in the process) you could try to search iteratively through a backup using regular expression search for the JPEG file header string. See this source: http://www.mikekunz.com/image_file_header.html .
A screenshot from my own attempt is attached to this post. I used Total Commander for Windows, and my case-sensitive regex search was for:
(?-m)\A\xFF\xD8\xFF\xE0..\x4A\x46\x49\x46
Some pdf files sneaked through, but otherwise the search produced fine quality results.
I'm glad I was able to get that far, as I may shortly be in the same situation as yours.
Regards.
Przemek
PS. If your case is one of an upgrade, previous backups of the courses under Moodle 1.9 - if you have any - should store all the files directly.