Google's Picasa photo editing and sharing software is quite easy to use .... But its "improved and enhanced" file navigation system has tripped me up so many times, that I thought I'd post a few obvious tips:

Picasa does not display images from your computer unless you have set Picasa to SCAN the folder that contains the images.  This is no problem if you always place folders of images in "My Pictures". 

If you stash the folder somewhere else, such as the Desktop, as you work on it, and the location is not included in your scan setup, Picasa is not going to display those images.

To access your scanning setup, click File -> Add Folder to Picasa.

The Folder Manager will open.  Note whether or not folders have been have been set to "Scan Once" or "Scan Always".  If the folder has been set to "Scan Once" and you added images to that folder since your initial scan, those images will not display unless you instruct Picasa to scan the folder again.

The little pointer triangle to the left of a folder name, indicates that the folder contains other folders inside.  Clicking once on the pointer triangle expands the folder.  Clicking on it again collapses the folder.

And finally, if you are comfortable with the familiar hierarchy model of files inside folders, and folders nested inside other folders, stick with the Tree View.  The Flat Folder View and the Simplified Tree View will seem strange and confusing.

To change your folder view, click View -> Folder View

PicasaTutorials.com by Geeks on Tour is an excellent Picasa reference with lots of tutorials.