Whenever you allocate memory in C\C++ you have to also free that memory when you are finished with it, I am assuming you are talking about an SDL_Surface when you mean an image. When you are sure you know longer need the surface, you may use SDL_FreeSurface to free the memory.

When you call SDL_Flip this is actually what redraws the background, all of the memory (surfaces) you used to draw to the screen will still be available when you are finished.

Yeah, so basically when you know for a fact that you will not need to use an SDL_Surface "image" to be rendered again in your game, if you are doing small tests, just free all of those at the very end of your program, if your project is bigger then a good resource caching system (Like in the book) would be good to have.