My question is, what is the difference between libSDL and libSDL-devel?
well libsdl is a library (maybe more)
libSdl-devel is the development package for this library (headerfiles etc)..
if you want to create an applications using libsdl then you need the development package.
if you get just a binary of the application it's probably dynamically linked to libsdl so you don't need the development package, but if you have the source of the application you may need the development package to be able to compile the application.
They have different packages for this because some people do not want the development packages, they do not have compilers installed and they think its just a waist of space.
If you downloaded and compiled and installed SDL by your self then you have the development files installed.
SDL_image is another package, as far i can see:
install it,then make sure you do as root
and try make clean before you do