Why are uploaded images stored in "secured"?

Thank you for the extensive reply. I am well aware that FC comes for free and I am full of respect for the developers who take on such an immense task and for sure I do not want to see you frustrated by my comment.

I am not complaining. I was just a bit disappointed from that set back. Of course I do backup the site prior to updating but not updating or downgrading is simply no option.

I had no time left to research what exactly went wrong. I just went trough all items with the image field and uploaded the images again. I don't think a bug report would have been of any use to you or other users since it is only a backwards incompatibility.

Thank you again for your time and effort.

Yes,
but giving us the broken URL at the time that images were not displayed, would give information us to what happened

or it could be that image field was working in "folder mode"
but checked both default value for code and default value for edit form of field configuration and it defaults to DB-mode