How to automatically crop and center an image

One solution is to use a background image centered within an element sized to the cropped dimensions.

Basic example

.center-cropped {
  width: 100px;
  height: 100px;
  background-position: center center;
  background-repeat: no-repeat;
}

<div class="center-cropped" 
     style="background-image: url('http://placehold.it/200x200');">
</div>

Example with img tag

This version retains the img tag so that we do not lose the ability to drag or right-click to save the image. Credit to Parker Bennett for the opacity trick.

.center-cropped {
  width: 100px;
  height: 100px;
  background-position: center center;
  background-repeat: no-repeat;
  overflow: hidden;
}

/* Set the image to fill its parent and make transparent */
.center-cropped img {
  min-height: 100%;
  min-width: 100%;
  /* IE 8 */
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=0)";
  /* IE 5-7 */
  filter: alpha(opacity=0);
  /* modern browsers */
  opacity: 0;
}

<div class="center-cropped" 
     style="background-image: url('http://placehold.it/200x200');">
  <img src="http://placehold.it/200x200" />
</div>

object-fit/-position

See supported browsers.

The CSS3 Images specification defines the object-fit and object-position properties which together allow for greater control over the scale and position of the image content of an img element. With these, it will be possible to achieve the desired effect:

.center-cropped {
  object-fit: none; /* Do not scale the image */
  object-position: center; /* Center the image within the element */
  height: 100px;
  width: 100px;
}

<img class="center-cropped" src="http://placehold.it/200x200" />

I was looking for a pure CSS solution using img tags (not the background image way).

I found this brilliant way to achieve the goal on crop thumbnails with css:

.thumbnail {
  position: relative;
  width: 200px;
  height: 200px;
  overflow: hidden;
}
.thumbnail img {
  position: absolute;
  left: 50%;
  top: 50%;
  height: 100%;
  width: auto;
  -webkit-transform: translate(-50%,-50%);
      -ms-transform: translate(-50%,-50%);
          transform: translate(-50%,-50%);
}
.thumbnail img.portrait {
  width: 100%;
  height: auto;
}

It is similar to @Nathan Redblur's answer but it allows for portrait images, too.

Works like a charm for me. The only thing you need to know about the image is whether it is portrait or landscape in order to set the .portrait class so I had to use a bit of Javascript for this part.

Try this: Set your image crop dimensions and use this line in your CSS:

object-fit: cover;

Example with img tag but without background-image

This solution retains the img tag so that we do not lose the ability to drag or right-click to save the image but without background-image just center and crop with css.

Maintain the aspect ratio fine except in very hight images. (check the link)

(view in action)

Markup

<div class="center-cropped">
    <img src="http://placehold.it/200x150" alt="" />
</div>

​ CSS

div.center-cropped {
  width: 100px;
  height: 100px;
  overflow:hidden;
}
div.center-cropped img {
  height: 100%;
  min-width: 100%;
  left: 50%;
  position: relative;
  transform: translateX(-50%);
}