Check out example codes for "transparent background css". It will help you in understanding the concepts better.

Code Example 1

img {
  opacity: 0.5;
}

Code Example 2

.opacity30 {
  opacity: 0.3;
  filter: alpha(opacity=30); /* For IE8 and earlier */
}

Code Example 3

background: rgba(255, 255, 255, 0.25);

Code Example 4

img {
  opacity: 0.5;
}

Code Example 5

body {
  background: #ffffff url("img_tree.png") no-repeat fixed right top;
}

Code Example 6

div {
  width: 200px;
  height: 200px;
  display: block;
  position: relative;
}

div::after {
  content: "";
  background: url(image.jpg);
  opacity: 0.5;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  position: absolute;
  z-index: -1;   
}

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