Tags and Descriptions
Unlike Flickr, Picasa Web has no support for tags. You can attach labels to individual images or an entire photo album in Picasa Photo software but that doesn't reflect in the web albums when you upload a photo. Both services allow you to add captions to pictures but Flickr automatically parses text and converts URLs into clickable hyperlinks.
Photo Notes
Flickr allows users to visually mark certain areas of the picture by drawing a marquee and attach a description and even a link to that area. For instance, in a group photo, you can mark your face and link it to your website. Picasa Web Albums have no such feature.
Photo Comments
Both services allow users to post comments on other pictures. In Flickr, you can also subscribe to your recent comments feed.
Picture Slideshows
Flickr creates photo slideshows using Flash while Picasa Web Slideshows are done in HTML and Javascript. Flickr Flash has more effect like zoomable thumbnails, dynamic hide-show features but Picasa Slideshows are more usable and easy to integrate in external websites.
Mark a Photo as favorite
Both services allow users to add any public picture to their favorite folder.
RSS Feeds
Both Picasa Web Albums and Flickr provide RSS feed for all photos from a user or selected album (or photoset). However, in Flickr you can subscribe to photos based on Tags.
Downloading Photos locally
Flickr allows download of individual photos while Picasa goes a step further and lets you download all pictures in a web album. However, Flickr provides various size options when you download a picture. We miss this in Picasa.
Support for Developers
Flickr exposes a nice API that has been used to develop some amazing web-apps like the Flickr Graph. Google Web Albums provides no API.
Support for Blogging
Flickr lets you directly publish a photo to a blog. You can do this from Picasa locally but not from the Web Album interface.
Where Google Picasa Web Album Wins:
Picasa provides an extremely easy program to upload photos online. You can visually select files and folders in Picasa, save them to the tray and upload all of them in one go. Picasa Slideshows are much easy to customize as they are done in HTML code.
Where Flickr Wins
Flicks is a more mature and robust application than Picasa Web Albums. Flickr supports tags and photo clusters which make the task of finding relevant photos so easy. Unlike Picasa, Flickr provides communities and photo pools where anyone can join and share related pictures. Flickr has an active support community and your Flickr related queries are answered almost instantly. Also, Flickr has an API for programmers.
Conclusion : Take the Flickr route. I would probably use Picasa Web Albums for storing picture backups but Flickr remains my favorite service until Google introduces tags, communities and more photo uploading options.
Firefox and IE7 users are more likely to be early adopters who have a keen interest in keeping up-to-date with the latest technology, said Don Leatham, director of solutions and strategy with patch management software vendor PatchLink.
Source: Labnol-blogspot.com