Whether you do image resizing (or other CPU-bound tasks) ahead-of-time or just-in-time is an extremely important design decision, that needs to consider the nee

Whether you do image resizing (or other CPU-bound tasks) ahead-of-time or just-in-time is an extremely important design decision, that needs to consider the needs of the application, available hardware resources, traffic patterns, etc - you shouldn't leave it to what is essentially random chance (maybe it's in the cache, maybe it isn't) without considering all of the above - unless you have a low-traffic blog that can get away with anything. Yes I was burned really badly by this in my young days ;)