That is quite a mystery to me, didierchretien. F-Stop just looks at your storage devices like all other apps if you were thinking that the images were stored in F-Stop somehow. One thing different is that we use our own media scanner vs using the built-in Android scanner that most apps use. Perhaps the issue is related to the built-in scanner not updating its database; however, I still think you should be able to see the files via USB. Maybe you could try rebooting your device?
You could also try using this app to see if it can refresh the media scanner:
https://play.google.com/store/apps/details?id=info.superkiki.mediascanner
I also found this info on media scanner issues related to a corrupt image file:
http://forums.androidcentral.com/verizon-lg-intuition/297308-media-scanning-problem-solved.html
