An interesting feature about Picasa is that it allows your to tag your photos with keywords.
That can become very handy if you have several GB of photos scattered in different folders and you want to search for a particular image or group of images.
To make use of this feature simply:
Select your photos (related theme or topic)
Do a CTRL + Left CLick for multiple selection,
CLick on the View Tab, scroll to Keyword (CTRL + k)
An new window shall open :
Add your Keywords in the text are and click on OK.
Searching for your pictures and photos
To search for a particular picture (which has already been tagged) simply type its corresponding keyword in Picasa the search bar.
The related images shall be displayed: