This is my first plugin, it's very simple and light.
You just need to copy the address of the pictures from Picasa album, then this plugin will help you modify the address to show in you posts. If the address of picture is not from Picasa, it won't do anything.
The plugin has been embeded into backstage editor, so you don't need to modify the theme files.
I just test it with 2.8. If anyone found bug on other lower version, please let me know.
Plugin Version: 1.0
Download: wp-picasa-image (download)