Imagefly is a proxy-based image processing service. Imagefly downloads, processes and returns the resulting image based on URL parameters. Subsequent requests for the same URL are returned immediately by our CDN.
Host URLs provide allow you to simply prefix your existing images when your own Imagefly hostname. Images will then be resized, optimized and cached on our CDN automatically.
Request signing is better suited for mobile apps or backend server integration. For example, you may want to resize images from a user's private Flickr account to deliver an optimally sized image in your mobile app.
See the Picturefill and Imagefly for examples.