Hi Polidor
Your files are on disk somewhere. Even if you delete files they are still there unless you overwrite them. Basically, there is a thing called a Master File Table (MFT) that holds all the details about every file on your hard disk. If you do delete a file, all that happens is that its entry in the MFT is removed, but the file still exists on disk, however Windows Explorer no longer sees it because it has been removed from the MFT.
Did you try that search thing I suggested in an earlier thread?
When Picassa saves a file which folder does it save them to? If you run the Picassa programme and try and save a file, what is the default folder where it tries to save them? That folder is likely to be the place where your missing photographs are.
downside