You need to initialize SDL_Image by calling  IMG_Init, before you can use any of SDL_Image's features.

Although both sport similar-looking APIs, always keep in mind that SDL and SDL_Image are completely separate libraries.  For instance, to check any SDL-related errors, use SDL_GetError; to check any SDL_Image-related errors, use IMG_GetError.

You need to initialize SDL_Image by calling  IMG_Init, before you can use any of SDL_Image's features.