Ok..

1. Go to the Default File Manager.
2. Choose 'Nokia N900'
3. Click on 'File Manager' at the top-middle of the screen to bring down the menu.
4. Choose ' new Folder'
5. Name it "Private"
6. Copy anything you want private to it.

The default image browser will not be able to see it, but you can still access it from the file manager. Just keep your fiancée away from your file manager. Or you can create the folder in lots of sub-folders to confuse her. Naming a folder private will ensure the tracker from looking into it. In the N900, the tracker is responsible for tracking images, music and video and add them to the libraries.