Exportation of Photostream on PC

I have sent photos from photostream on my computer, and plan to upload the photos to Facebook from the Photostream photo (right now, I prefer not to upload it straight from the iPhone). The photos, however, will not allow me to do so, and I can not copy the files, and can only view them. Is there a setting that I need to change, or is it more complicated than that? Any help is appreciated! 🙂