You can optionally add three kinds of identifiers to selected files to make them easier for you to find in searches: captions, star ratings, and keywords.
Add captions. Every image can have a captiontext that identifies the photo's subject, when/where it was taken, or something funny, for example.
Open the image for editing by double-clicking its thumbnail in the Lightbox.
If Make a caption! isn't visible beneath the image, click the Show/Hide Caption icon in the left-hand corner beneath the image.
Click the Make a caption! text beneath the picture and type a caption, such as Evan and high school friends.
Mark photos with a star. You can denote special photos by marking them with a star. What the star means is up to you. It could be used to mark your favorite pictures or only high-resolution ones, for example. A search option (discussed in Chapter 3) allows you to display only starred pictures.
To mark the selected photo(s) in the Picture Tray, click the Star icon. (Note that the Star icon is present both when working in the Library or when editing a picture.) To remove the star from selected images, click the Star icon again.
Assign keywords. Another way to make it easier to find pictures with a Picasa search is to assign keywords to them. A keyword is a single word that describes an aspect of the picture, such as its subject. You can assign one or multiple keywords to an image.
Assign or remove an image's keywords in this dialog box.
Select an image thumbnail in the Lightbox or open the image for editing.
Choose View > Keywords (). The Picasa: Keywords dialog box opens.
To assign a new keyword to the image, type it in the Add Keyword box and click the Add button. The keyword is added to the Keywords list for the picture.
To create additional keywords, repeat Step 3. When you're done, click OK.