I'm not sure if I understand your question but here goes:
I think you want the sprite to wave over an angle back and forth:
Not this is pseudocode (as you didn't state the language):
a = Sin(t)*amplitude;
Position.X=StartPosition.X + Cos(angle) * a;
Position.Y=StartPosition.Y + Sin(angle) * a;
t controls the waving motion; the
wavespeed variable is a small value like
0.01 (note that it should slowly range from 0 to TwoPi).
amplitude is the maximum you want your sprite to move,
a is calculated as the amplitude for that frame.
angle is the angle of the axis you want your sprite to wave along.