Picasa possesses a wide array of filters and effects to match your creative needs. Black and White, Saturation, Focus .... most of the tools need to make some nice image editing are present.
Before modifiying any of your pictures, it better to add them in a new album (the orignal files will remain untouched).
Read this FAQ for more info about this topic.
Double click on the picture you want to modify.
This will bring up the "Image editing pane"
This section is divided into 3 tabs.
Basic fixes: as the name suggest you can perform basic manipulations, such as "Cropping", "Auto contrast" tc
Tuning: Manage light fill, shadows tc
Effect tabs: contain the various effects, that's what we have been looking for. Hover your mouse cursor of the different effects to get some information about what it's all about.
In this part, you should get a preview of the picture and the modifications made along.
Double click on a the effect you want to add and get a preview.
You can adjust the light/Shadows effects in the Tuning tab.
Once done ,click on the "Back to Library" button.
Simply click on the "Save Changes " button to finalize your work.
Picture credits Common creatives.