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Web based photo sharing sites are websites where you can upload, share and access digital photos and images. One of the most famous is Flickr (which has it's own page here on the toolkit site).
Another option, possibly not as popular within the development community, but offering some unique qualities, is Picasa , for image management and sharing. Picasa is available both as a downloaded version to run on your computer and manage images files locally and also as a web version for sharing images. The desktop version is useful for those who are not always connected to the Internet. It was developed by Google and integrates with other Google products such as Blogger, etc.
Here is a brief video explaining photosharing.
Photo Sharing in plain English: http://www.commoncraft.com/photosharing