Attached Link: http://hdri.3dweave.com/library/

This site I've linked to explains much better than I can what hdri is and the physics involved in the images. It even explains why the image is LDRI (shifted blue). The great thing about the dancing bear...ops, The great thing about hdri or in this case ldri is that it ads a 360 realistic reflection to metal or glass objects in your scene. Here is the same image with a chalace inserted. The chalice is about 180k and the hdr image is about 340k. Imagine building an entire town square with 20 buildings, a few poser characters 200+ plants an outdoor cafe just to produce this same shot. It would take gbytes of memory. In bryce you would also be waiting while the rays are calculated off each window, down the alleys and back to the camera. With the hdr you're not really calculating the path of lightrays as they glance off buildings, You're just calculating the bounce off the sphere and back.