#gallery img {margin: 4px; border: 1px solid #aaa}

#thumbnailContainer img {
	margin: 4px;
	border: 2px solid #333;
	position: relative;
}

#thumbnailContainer img:hover {margin: 4px; border: 2px solid red; opacity: 0.5}

#thumbnailContainer img:hover::before {content: 'Remove';
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  color: #fff;
  font-weight: bold;
  text-align: center;
  background-color: rgba(0, 0, 0, 0.5);
  width: 100%;
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
}