Erm, and who would actually bother? Sorry, but let me be straight: This is pretty much nonsense. If the file already requires to use PSB it is too large for any normal viewer even if it were to be saved as a JPEG, which in theory may support up to 64000x64000pixels, but fails from the fact that most viewers cap of at 16000 due to memory requirements and other tech stuff. And even if it were possible - it's unlikely that people download 500MB JPEGs. Again, you are trying to do something which for simple practical reasons will never work. You created an Edsel. If at all, you need to find a service that offers adaptive streaming of multiple resolution tiled images similar to how NASA or Google do it with the maps and space photos or one of those panorama services that use specific Flash or Java applets, but you will never ever be able to just throw one huge file out there and hope anyone even cares to look at your humongous creation.
Mylenium