stb_image is a single header, freeware, image decoder. On the other hand, libpng is a dedicated library that has seen severy different ABI (12 / 14 / 16) and is update frequently for security reason. Removing a dependancyby default looks good to me, even if libpng is probably more optimized than stb_image, I think in the end it makes very little difference in performances for SDL_image use.
Yes it will work on Pyra. stb_image works fine there (I use it on the "Prototype" dbp IIRC, maybe more...).