After the installation, go to Wordpress Settings -> ESPR!T Picasa and set:
the username of your Google Picasa album.
size of the thumbnails which will be included in your posts or pages
size of the large version of images which will be displayed after clicking on thumbnail
While editing the post or page, click on Picasa icon on top of the editor window (in the same place ase are the other default media icons located). New window with list of albums will be opened, click on the album for list of its images and after clicking on image it will be inserted in your post or page.
(It is recommended to save the page/post you are editing first, as it will get an post id assigned - the ESPR!T Picasa uses such id as a reference for generating Lightbox code)
Credits
Thanks go out to:
Bogdan Necula for his Picasa LightBox 2 plugin which I have used as an inspiration and completely rewrote it for newer versions of Wordpress
Cameron Hinkle for his Lightweight PHP Picasa API, which I have used after realizing that the Zend GData libraries are piece of crap