1. Go here, sign in to Flickr and copy the email address provided.
2. Open Picasa, select the pictures and click on the "Email" button from the bottom of the window.
3. In the subject type the title of the picture followed by the tags.
In the body, type the description of the photo.
Subject example:
Sunny Valley tags:trip july "lucy cradle"
As Picasa is integrated with Gmail, the best thing is to use the Gmail account. Next time you want to send photos, just type Flickr in the "To" field and the autocomplete function will help you find the email address where to send the pictures. Remember to keep it private.
This method has some limitations: if you send more than one photo, all the photos will have the same title, description and tags, so you'll have to edit them from the Web interface. Flickr says there's no limit for the number of photos sent by email, but don't send too many, especially if they are high-resolution photos.
{ Based on a post from Digital Media Minute. }
Related:
Picasa 2.5 review
Online image editor for Flickr