[Melraidin]

Generating many conversions for all photos incurs both storage and computational overhead that may never pay off. By only generating the required images on the fly for photos we avoid this overhead while allowing more flexibility. If watermarks or attribution change in the future it's easier to have the resize service in place to handle this as images expire from the CDN than to do a batch job to pre-process the updated conversion requirements for all existing photos.

Also note that photos are stored indefinitely, so any additional permanent storage (new conversions) would add additional storage costs, miniscule as they might be, forever.