can't test at the moment but try adding this at the beginning of your CSS:

@keyframes float {
  0% {bottom: 0px;}
  50% {bottom: 50px;}
}

and this under body::before:

  animation-name: float;
  animation-duration: 4s;
  animation-iteration-count: infinite;

you may adjust the "bottom" (under keyframes) and "animation-duration" values to your liking

easiest way i think to do that would just be to make the image float up and down as a gif but i also. barely fucking know css so don't trust me 😭😭😭