SDL_DisplayFormat returns null for no reason

[Tainor]

I'm learning SDL but I have run across a problem which I can't understand. For some reason, whenever I try to convert an image to the display format it returns a NULL pointer. I've checked the linker and order of functions and everything is right.

Curiously, if instead of trying to optimize the image I just load it and don't optimize, the image appears. Optimizing it does not.

#include<SDL\SDL.h>
#include<SDL\SDL_image.h>

SDL_Surface *cargarImagen(char* imagen);
void aplicarImagen(int x, int y, SDL_Surface *origen, SDL_Surface *destino);

int main(int argc, char *args[])
{
bool quitProgram=0;
SDL_Event event;
SDL_Surface *screen;
SDL_Surface *mensaje;
SDL_Surface *fondo;
SDL_Init(SDL_INIT_EVERYTHING);
mensaje=cargarImagen("mensaje.png");
fondo=cargarImagen("fondo.png");
screen=SDL_SetVideoMode(640,480,32,SDL_SWSURFACE);
while(quitProgram==0)
    {
    SDL_PollEvent(&event);
    if(event.type==SDL_QUIT) {quitProgram=1;}
    if(event.type==SDL_KEYDOWN)
        {
        aplicarImagen(100,100,fondo,screen);
        SDL_Flip(screen);
        SDL_Delay(1500);
        }
    else {aplicarImagen(100,100,mensaje,screen);}
    SDL_Flip(screen);
    }
SDL_FreeSurface(mensaje);
SDL_FreeSurface(fondo);
SDL_Quit();
return 0;
}


SDL_Surface *cargarImagen(char* imagen)
{
SDL_Surface* loadedImage = NULL; 
SDL_Surface* optimizedImage = NULL; 
loadedImage = IMG_Load(imagen); 
if( loadedImage != NULL ) 
    { 
    optimizedImage = SDL_DisplayFormat( loadedImage ); 
    SDL_FreeSurface( loadedImage );
    } 
return optimizedImage;
}


void aplicarImagen(int x, int y, SDL_Surface *origen, SDL_Surface *destino)
{
SDL_Rect aux;
aux.x=x;
aux.y=y;
SDL_BlitSurface(origen,NULL,destino,&aux);
return;
}

Is there anything wrong with the code?

[lafigueroa]

move the statement

screen=SDL_SetVideoMode(640,480,32,SDL_SWSURFACE);

before the function calls to cargarImagen.

[Tainor]

So the function returned null because there was no screen surface at all to convert to. I'll have to look at it more carefully next time. Thank you, it works now =)