To add to Phil's comments. I can't think of any resson loading a background picture would stop your character from posing. I have done this many times without any problem. Of course when you save your work, if you want to edit it in future you need to save it as a pz3 or pzz, you can't pose a graphic file such as psd or jpg.

Poser is not particularly suted to making props, you are more or less limeted to scaling, morphing, and combining primiteves, though you can also delete facets in the Grouping Tool. To cerate more complex props you really need a modlling program such as the free Wings, Anim8tor, or Metasequoia LE.