How to make an image center (vertically & horizontally) inside a bigger div

Personally, I'd place it as the background image within the div, the CSS for that being:

#demo {
    background: url(bg_apple_little.gif) no-repeat center center;
    height: 200px;
    width: 200px;
}

(Assumes a div with id="demo" as you are already specifying height and width adding a background shouldn't be an issue)

Let the browser take the strain.

another way is to create a table with valign, of course. This would work regardless of you knowing the div's height or not.

<div>
   <table width="100%" height="100%" align="center" valign="center">
   <tr><td>
      <img src="foo.jpg" alt="foo" />
   </td></tr>
   </table>
</div>

but you should always stick to just css whenever possible.

I would set your larger div with position:relative; then for your image do this:

img.classname{
   position:absolute;
   top:50%;
   left:50%;
   margin-top:-25px;
   margin-left:-25px;
}

This only works because you know the dimensions of both the image and the containing div. This will also let you have other items within the containing div... where solutions like using line-height will not.

EDIT: Note... your margins are negative half of the size of the image.

Use Flexbox:

.outerDiv {
  display: flex;
  flex-direction: column;
  justify-content: center;  /* Centering y-axis */
  align-items :center; /* Centering x-axis */
}

in the div

style="text-align:center; line-height:200px"

Vertical-align is one of the most misused css styles. It doesn't work how you might expect on elements that are not td's or css "display: table-cell".

This is a very good post on the matter. http://phrogz.net/CSS/vertical-align/index.html

The most common methods to acheive what you're looking for are:

• padding top/bottom
• position absolute
• line-height

Typically, I'll set the line-height to be 200px. Usually does the trick.

div {
  position: absolute;
  
  border: 3px solid green;
  width: 200px;
  height: 200px;
}

img { 
  position: relative;
  
  border: 3px solid red;
  width: 50px;
  height: 50px;
}

.center {    
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  -ms-transform: translate(-50%, -50%); /* IE 9 */
  -webkit-transform: translate(-50%, -50%); /* Chrome, Safari, Opera */
}

<div class="center">
  <img class="center" src="http://placeholders.org/250/000/fff" />
</div>

Related: Center a image

If you know the size of the parent div and the image, you can just use absolute positioning.

I had this issue in HTML5 using CSS3 and my image was centered as such within the DIV... oh yes, I can't forget how I had to add the height to show the image... for a while I wondered where it was until I did this. I don't think the position and display are necessary.

background-image: url('../Images/01.png');    
background-repeat:no-repeat;
background-position:center;
position:relative;
display:block;
height:60px;

Here try this out.

.parentdiv {
 height: 400px;
 border: 2px solid #cccccc;
  background: #efefef;
 position: relative;
}
.childcontainer {
 position: absolute;
 left: 50%;
 top: 50%;
}
.childdiv {
 width: 150px;
 height:150px;
 background: lightgreen;
 border-radius: 50%;
 border: 2px solid green;
 margin-top: -50%;
 margin-left: -50%;
}

<div class="parentdiv">
  <div class="childcontainer">
     <div class="childdiv">
     </div>
  </div>
</div>