You can use the search function in Picasa to locate your photos based on a number of criteria. Search for photos using the search box or by using preset search filters:
Picasa can find any text associated with a photo. This includes the following:
Filenames
Captions
Tags
Folder or album names
Camera maker
Date (i.e. 'November' or '2008')
Enter a term in the search box in the upper right-hand corner of your Library. As you type your search term, Picasa refines the results. To narrow your search, include multiple words in your search criteria.
NOTE: For a more detailed discussion of Search Terms and Key Words, see Search for pictures in Picasa
You can search for your photos using preset Picasa filters. 5 search filter buttons are located to the left of the search box in your Library.
Starred photos: Shows all starred photos.
Uploaded photos: Displays photos uploaded to Picasa Web Albums.
People: Displays pictures where Picasa has detected a face.
Movies: Presents all movie files.
Geotags: Shows you all photos with geotags applied.
These filters can be combined with each other and also with a keyword search. For example, you may want to know which photos you've uploaded to Picasa Web Albums with the tag 'flowers'.
You can also try an experimental feature that lets you search for photos that contain a certain color:
Click the Tools menu.
Select Experimental.
Click Search for.
Select the color you'd like to search for.
Picasa will display all photos where your selected color is detected.
All photos included in the search results will be grouped together in a temporary album, accessible in your Folder list under the Albums collection. You can perform any normal action (i.e. upload, email, BlogThis!) on the photos included in this album. The search results remain until you reset the search. You can reset a search by clicking the Back to View All button in the green bar at the top of the Library.