Adding SDL_image to the mix wasn't as simple as just copying it to the jni/ directory and calling "ndk-build". Doing so yielded this error:
jni/SDL_image/IMG_jpg.c:34:21: fatal error: jpeglib.h: No such file or directory
Logically enough, SDL_image needs libpng, libz, and libjpeg to build.Fortunately - as a time saver - you can still grab copies of these on on the libSDL website. You can then copy jpeg/ and png/ into the jni/ folder and: "ndk-build clean", followed by "ndk-build" to rebuild.
EDIT: It turns out there's a more recent framework here. All you will have to do is update the Android.mk in jni/src with the libraries sources you use, and be sure to load the right libraries in SDLActivity.java, of course.