I see there's a new product in the RMP that offers HDRIs for use in Poser. From the description:

"Each HDRI is available in 16, 8, 4, and 2K resolution. The 16K HDRI is very useful if you want to render it both as a light emitter and a background. As the image is wrapped around a sphere, more resolution is needed than a flat background image in order to preserve detail. With a 16K HDRI you will be able to retain a sharp and clear background even for high resolution render. However, it is somewhat taxing on graphics resources, so if you are rendering a smaller image, use a narrow depth of field, or only need it for the lighting, the 4K or even 2K is more than sufficient, the lighting quality will be pretty much the same."

I've played around a little with the envirosphere and EZ-dome, but I have a hard time finding suitable images for them.