For background images you can use any supported image file type which includes JPG, BMP, TGA, PNG, and TIFF. TIFF is definitely a good option, although not all variants of TIFF are supported. You would then replace the Background Object's Background Shader with an Image Map Shader, set it to Spherical Mapping. That should get you started and from there you can experiment to see what works best for your needs.
- Oshyan