A spherical image is one that can be wrapped around a sphere without distortion. Think of the Carrara universe as a giant invisible sphere. If you were to place an image in the Background in the Scene’s settings, it would wrap around the universe. If it’s a standard photograph it would appear distorted. A spherical render would appear normal. The cityscape in the samples I posted is a spherical render. The background has the advantage of being able to be reflected, used as a light source, etc. The disadvantage is that it cannot be seen until you do a spot render or render, and normal images are distorted as mentioned above. The best place to use a normal image is in the Scene’s backdrop. The backdrop is not reflected and cannot be used as a light source.
HDRI (High Dynamic Range Images) can be used as a light source when placed in the Background and the Skylight box is checked in the Render Room. You can also use standard images as light sources as well, it’s just not lit with as great a range of colors, etc. You can also use flat colors, gradients, etc. When used as a light source, HDRIs and the like are considered IBL (Image Based Lighting). Brighter areas of the image give off more light, and darker areas less light. The light is more of a diffuse light with soft shadows. If you want the light of an HDRI, but don’t want the image visible, you can use an image or color in the Scene’s Backdrop.
Here’s the spherical image I used. It’s one I rendered using Carrara’s spherical camera, and I’ve made it available without restrictions here:
There’s also a night version.