Not sure if you’re asking about how I have the cat in the corner or the animation for it so I’ll just give you both codes

body::after {
    content: '';
    position: fixed;
    bottom: 0;
    left: 0.5rem;
    width: 12rem;
    height: 12rem;
    background: url(your image link);
    background-size: contain;
    background-repeat: no-repeat;
    z-index: 1;
    pointer-events: none;
    animation: float 4s infinite;
}

@keyframes float {
    0% { transform: translateY(0); }
    50% { transform: translateY(-1rem); }
} 

If you don’t want the animation then remove the animation and @keyframes property, otherwise leave it be and change your image link to a link to your image