This article follows steps using Smith Micro Poser 7. Several of these steps become easier in newer versions of Poser.
Importing in a spherical map isn't an exact science with some versions of Poser. There are a few ways to tweak your image.
Make sure your render size matches the ratio of your background. You may have further adjustments to make as Poser 7 and earlier have limitations on background sizes greater the 4096 pixels.
Try adjusting the Main Camera scale, increasing it to 200 or 400%.
Poser Pro 2012 includes and optimization function for using HDR images within Poser. Read the article Optimizing for HDRI in Poser Pro