Picasa Web Albums lets you see detailed visitor stats for your photos– just go to Settings page, enable "photo tracking". "Help center explains that you need to create new Google Analytics account to monitor Web Albums photo traffic. After creating, find the account ID: UA-xxxxxxx-y and enter it in the Analytics tracking code box from PWA. "Once the setup is complete, just sign in to Google Analytics and click View reports to see visitor stats for your photos. It can take up to 24 hours for Analytics to detect your tracking code.""
A similar option is available for Google Docs, but only for published documents. While this feature is useful, it's not very easy to use and it's not properly integrated with Google Docs and Picasa Web.
Also, Albums added an option that "Picasa Web Albums are now compatible with Google Profile! Now when you use PWA, you can use same profile name and photo that you use on your Google Profile. Your PWA will link to your Google Profile, and your profile will link to your public albums," informs Google's photo service.
After linking your Google Profile with PWA, you can edit profile and remove the link to your public albums. If you've previously used an alias to hide your Gmail address from the URL, you can no longer use it after merging profiles. The only option you've is to use same ID number from Google Profiles. Picasa Web's code calls this feature "merged profiles softlaunch", which suggests that users aren't required to merge profiles, but that'll change in the future.