Thanks all for your real-time replies, I greatly appreciate that! Now some update on the story:
1. The good news is that I have deleted the folder from the backup drive, not from my main drive, so I could successfully reduce disk activity.
2. This made me think that I have cut the folder from the main drive and pasted it onto the backup drive instead of having copied it (ctrl + x instead of ctrl +c). Then I must have deleted it when I tried to make a new backup of that folder as I needed to free space for the new one.
3. However, this doesn´t make any sense because if the folder hasn´t been present, how could I have added new files and hence have to make a new backup
4.
The answer: Unfortunately, this isn´t some kind of schizophrenic question as the real cause for the whole @$%&/:. hassle turned out to be quite surprising. The folder in question was the default picture subfolder in the main user folder (e.g. Marin). As it was now missing (due to having been deleted) I created a new one again called Pictures. It was successfully recognized by Vista (Vista even changed the icon and added the default shortcut to "sample pictures"). To my surprise the same folder suddenly appeared on the backup drive in the ultrabay. Then I made a little experiment deleting that folder and the result was that the Pictures folder in Vista main user folder disappeared as well. I guess this experiment explains how my pictures had disappeared...
5. I have no explanation for that as I don´t use any sync software (not even M$ briefcase), just plain copy-paste.
This was the bad news. But the worst news is that using trial copies of recovery software like what was needed here in real-life situations (i.e. not for experimental purposes) is a very bad idea. For it would restore the files, but only the unlocked version would allow to save the restored files (anywhere). And since in most cases this kind of restoration is only one way, so I totally screwed up the restoration of anyway low-quality rests. I wish I was more careful regarding trial limitations.
Anyway, looking at the scan results and the theoretical abilities of each tool I was particularly impressed by PhotoRescue as it even allows to restore an image from "pieces" and by DiskInternals Uneraser tool. Boot CDs weren´t an option for me since the ultrabay is occupied by the HD in question.
Thanks for all hints and recommendations and hope my experience will help other lost souls
Marin
PS: I don´t know what to expect from Vista next...