Picasa Web's search was a great feature that allowed you to restrict the results to your photos, the photos from your favorite users or all the public photos. Google included a lot of advanced search features, so you could sort the results by date or relevance, change thumbnail size, find Creative Commons photos, restrict the results to panoramas or portrait photos and start a slideshow from the results.
"Picasa Web Albums searches across tags, captions, album titles, album descriptions, and album locations to deliver you relevant photos and videos. Choose one of the advanced search options on the left to refine your results. Among your options are faces (with faces or without), aspect ratio (Landscape vs. Portrait vs. Panoroma), size, format (photos vs. video), camera type (Nikon, Canon, etc.), Creative Commons license type (copyright status), and the order (by date or by relevance)," informed Google.
Unfortunately, Picasa Web search no longer exists and it now redirects to Google+ Photos, which lacks all the advanced features I mentioned.
If you've joined Google+, you can see results from your photo albums and from the albums created by your circles:
If you're not a Google+ user, Google shows a list of Google+ posts that include photos:
{ Thanks, Florian K. }