On the Mac you need to add the .br6 extension when saving the Bryce scene (or do it in the Finder).
Broken Save Image As is a known bug.
I can load 16-bit non-interlaced PNGs created in Photoshop CS using the Terrain Editor's Picture button. 16-bit TIFFs don't work for me.
I can also load 8 bit PNG, PSD, TIFF and JPG image textures.
Having said that the license problem is disturbing - that could point to a serious  install problem which I'm not sure how to fix.