Scaling an image upload service


First of all I want to to say that this is an extremely interesting and informative website. i have enjoyed reading the various posts on how the big sites scale to meet the needs of their customers.

The service we are developing is a webcam service. The client application sends images to the server via HTTP POST and they are saved in folder specified by the users id. When a new image is sent to the server it will overwrite the current image.
Users can then view the images via our web server.
Ideally we want the images to upload as quickly as possible and allow users to view them as quickly as possible.

Would I be correct to assume that when the number of uploading clients exceeds the capability of the server the only way to scale is to add more hardware.

Also I assume that to use HTTP accelerator caches will not speed up viewing the images as the new images will invalidate the cache.

I appreciate any input on the subject.