Change background image opacity

You can't use transparency on background-images directly, but you can achieve this effect with something like this:

http://jsfiddle.net/m4TgL/

HTML:

<div class="container">
    <div class="content">//my blog post</div>
</div>​

CSS:

.container {  position: relative; }

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

.content {
    position: relative; 
    z-index: 2;
}​

Nowadays, it is possible to do it simply with CSS property "background-blend-mode".

<div id="content">Only one div needed</div>

div#content {
    background-image: url(my_image.png);
    background-color: rgba(255,255,255,0.6);
    background-blend-mode: lighten;
    /* You may add things like width, height, background-size... */
}

It will blend the background-color (which is white, 0.6 opacity) into the background image. Learn more here (W3S).

Try doing this:

.bg_rgba {
  background-image: url(https://picsum.photos/200);
  background-color: rgba(255, 255, 255, 0.486);
  background-blend-mode: overlay;
  width: 200px;
  height: 200px;
  border: 1px solid black;
}

<div class="bg_rgba"></div>

worked in my case. You can reduce or increase the background-color alpha value according to your needs.