img.lazy {
  opacity: 0;
  transition: opacity 0.3s;
}

img.loaded {
  opacity: 1;
}

img[data-src] {
  background: #f8f8f8;
  position: relative;
}

img[data-src]:before {
  content: '';
  position: absolute;
  top: 50%;
  left: 50%;
  width: 40px;
  height: 40px;
  margin: -20px 0 0 -20px;
  border: 4px solid #e3e3e3;
  border-top: 4px solid #007bff;
  border-radius: 50%;
  animation: spin 1s linear infinite;
}

@keyframes spin {
  0% { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}

img.loaded:before {
  display: none;
}