I'm curious to know how people feel about offline (pre-transformed) vs. on-demand transformations. Are there any HN'ers out there that have worked on a site with a large set of images, and have an opinion on this? Adobe's Scene7 product works in an offline mode as far as I can tell, and seems to have captured a large segment of retail companies with product catalogs.
I had worked for a social network, and our system provides a function to let user upload their photo then transform it to some fixed size of original one. We did have pre-transformed and on-demand too.
Pre-transformed for the image that's most viewed by user, like new feed's photo (720x720), large photo (1024x768), and the origin one (if user's screen is detected as big screen), we have to resize it asap. Other sizes, like thumbnail, we do on-demand transform using nginx resize filter plugins, and caching using varnish and/or traffic server.
That system have been working well until this time.
I would say on-demand transformation is good idea, since you don't have to store resized-image that's never viewed by any user, so you save your storage. But that idea must be implemented well, very well if you're going to serve million users.