As with Facebook and Flickr, iPhoto '11 integrates Picasa Web Albums, letting you upload photos from your library within iPhoto. It's easy to select a group of photos to upload to a new or existing Picasa album, and I will show you how.
First, download the Picasa Web Albums uploader. It will add a Picasa export option in iPhoto. Next, highlight a group of photos or an Event and from the top menu bar, select File > Export. On the Export window, click on Picasa Web Albums from the top row of options. (The first time you do this, you'll need to enter your Google username and password to log in.) You can then choose to add the photos to an existing album or create a new album, giving it a title and description. You can also choose to make the album public or private, whether to use iPhoto keywords as photo tags on Picasa, and whether to scale large photos for better quality or a faster upload. Lastly, in the lower-left corner of the export window, it displays how many photos you've selected.
When you have everything set, hit the Export button. After iPhoto has uploaded the photos to Picasa, it'll ask you whether you'd like to view the Web album. Click View and the album will open in your default browser.
That's it! It couldn't be easier to upload photos to Picasa Web Albums from iPhoto '11.