Use SPDEF to define the sprite and associate it with a definition number.
SPDEF DN,X,Y,WITH,HEIGHT,CENTERX,CENTERY,EXTRAS
Example:
SPDEF 0,0,0,16,16,0,0,&b1 OR &b1000
SPSET 0
&B1 means that the sprite is visible.
&B1000 means that the sprite is flipped(Horizontally).