As far as I can tell you can't add a "background" to Studio via an image, you'll need to use either one of the free scene props that are available from either Nerd or RDNA or someone (I think I still have one I made on my drive somewhere) or the cyclorama from Daz. Can't answer your second question as I don't have P5, sorry. At a guess, if you mean through a blender to say the textures are all messed up, that's not surprising. Poser 5 texture settings are vastly different to Studio and Poser4 which Studio currently imports from.