Hello, Alain
I'm truly sorry to hear about your problems, but...there are some weird things here...
1. you're saying you changed the database prefix using the plugin, so far so good, can you please tell me the result of that action? Were there any errors? The plugin will let you know with a message right below the form telling you whether it successfully changed the db prefix or if there were errors doing it.
2. you're saying that after the db prefix change you cannot see images in your website...and after you upload the backup files the images were showing again... now that is the most weird thing here, because changing the database prefix has nothing to do with the file system, so I fail to understand how the db prefix change could affect the file system....
In conclusion, if there were no errors reported by the plugin when you changed the db prefix then nothing should be broken on your websites, so these problems could be related to something else...
Looking forward to hearing from you
Regards,
Costin