You can subscribe to Flickr or Picasa Web Albums’ RSS feeds by copying the RSS link (at the bottom right of the album page if the owner of the album has set the corresponding pref in his Flickr/Picasa Web profile), switching to iPhoto, selecting File » Subscribe to Photo Feed, and pasting the link.
You will see a new item in the left hand Library window with the RSS album’s name. Clicking on that name will show you all the pictures in that album in the main iPhoto window. You can then drag drop pictures from the Picasa/Flickr album into your main iPhoto library.
Enjoy !