SDL_SetColorKey man page
SDL_SetColorKey — Sets the color key (transparent pixel) in a blittable surface and RLE acceleration.
int SDL_SetColorKey(SDL_Surface *surface, Uint32 flag, Uint32 key);
Sets the color key (transparent pixel) in a blittable surface and enables or disables RLE blit acceleration.
RLE acceleration can substantially speed up blitting of images with large horizontal runs of transparent pixels (i.e., pixels that match the key value). The key must be of the same pixel format as the surface, SDL_MapRGB is often useful for obtaining an acceptable value.
If flag is SDL_SRCCOLORKEY then key is the transparent pixel value in the source image of a blit.
If flag is OR'd with SDL_RLEACCEL then the surface will be draw using RLE acceleration when drawn with SDL_BlitSurface. The surface will actually be encoded for RLE acceleration the first time SDL_BlitSurface or SDL_DisplayFormat is called on the surface.
If flag is 0, this function clears any current color key.
This function returns 0, or -1 if there was an error.
SDL_BlitSurface, SDL_DisplayFormat, SDL_MapRGB, SDL_SetAlpha