I finally managed to fix the problem by doing what Alee indicated on the previous page. I created a backup on my laptop (DCIM and Pictures), then disconnected the N9, deleted all content within the Gallery application, reconnected it to the laptop and deleted the DCIM and Pictures on the N9, rebooted, pasted the backups on the N9, rebooted, still nothing in the Gallery, so I ran gallerybugx in terminal and rebooted again, and voilà!

I'm sure some steps were unnecessary here, but at the end of the day it worked and did not cost much time. Thanks a lot to all of you.