SDL_GFX question

Dear community,

Please could someone post a small snippet of how to correctly use
SDL_imageFilterAdd (unsigned char *Src1, unsigned char *Src2, unsigned
char *Dest, int length); with SDL?

I would like to use this command to blit on the background surface in
sdl. I am hoping someone is kind enough to supply some source showing
how best to do this. Ideally I would have the source being an sdl image,
and blend it with the background of the same size as the image and draw
it onto the background surface.

However I have really not got a clue how to get it working despite
looking at docs! please help!