Why are the high resolution files TIFFs, and the medium and low resolution files JPEGs?
Because the high-resolution files are likely to be used for high-end print applications, creating a JPEG may reduce overall quality. This is not necessarily such an issue for the applications for which 5Mbyte images are likely to be used. Also, JPEGing the 50Mbyte image without degrading it would only reduce the file size to around 25-30Mbyte, and this is still very big. JPEGing the 18Mbyte file can get images down to around 5Mbyte, which is acceptable for Web download.
