How do I uninstall Picasa?

This depends in large part on how you installed Picasa for Linux.
It comes in several flavors; read on to see if you can find one that matches your system.

Also note that you'll want to remove the ~/.picasa directory (notice the dot) for each user that used Picasa; that directory contains per user settings and configuration information. The uninstall routine will try to do that for you, but if you uninstall as the root user, it will be unable to do that for other users, so you may wish to do that step yourself.