Microsoft Live Labs recently released Seadragon.Com. Using Seadragon, you can share any size image on the net as a zoomable photo. You can then share the image on Facebook, Twitter, or embed it directly on your blog or any other site.
The picture viewer is developed using the Silverlight Deep Zoom technology because of which the image loads quickly irrespective of the size. The viewer also adjusts dynamically to the layout of your page, or can be easily customized to a specific size so that any size image fits perfectly on your site. And since Seadragon.com uses a dual-mode Silverlight & AJAX control, it works on virtually any browser--automatically using Silverlight to provide a smoother experience and better performance when available, and defaulting to a scripted viewer when it is not.
You can learn more about Deep Zoom in Silverlight Deep Zoom Documentation.