UNDERSTANDING HOW PICASA WORKS
Picasa is a free program owned by Google
He is a friendly little fellow - once you get to know him 
All photo editing programs give you the tools you need to edit, make changes to your photos and much more besides.
When in other programs you Browse in your picture folder for the photos you want and then open them into that program.
Imagine how slow that would be if you were making collages! :-(
On the other hand, Picasa by-passes the Browse step
and shows you your pictures in their folders immediately and
so they are there ready for you without any further searching! ;-)
Your pictures are not IN Picasa, they are ON your hard drive.
Picasa shows them to you and allows you to edit them,
manage your folders, make collages and so on. 
You have complete control over the folders you allow
Picasa to scan for photos.
Go to Tools Folder Manager
And mark with the blue circle your Picture Folder and anywhere else
there are photos
Everything else mark with a red cross.
Scan always.
Edits are kept on the Picasa data base and while you stay within Picasa
There is no need to save anything.
HOWEVER,
If you want to use the edited photo in another program, you must
Save it or Export whilst in Picasa so that you can see and use
the edited version of your photo on your hard drive.
I have written a tutorial and it is available in PDF.
Margaret Carrington
Here are some helpful sites
https://sites.google.com/site/picasaresources/faqgeeksontour.com/ The Geeks Chris & Jim Guld tour the US in
http://picasatutorials.com/ their RV holding computer and Picasa
seminars for fellow travelers.