How can I make all images of different height and width the same via CSS?

Updated answer (No IE11 support)

img {
    float: left;
    width:  100px;
    height: 100px;
    object-fit: cover;
}

<img src="http://i.imgur.com/tI5jq2c.jpg">
<img src="http://i.imgur.com/37w80TG.jpg">
<img src="http://i.imgur.com/B1MCOtx.jpg">

Original answer

.img {
    float: left;
    width:  100px;
    height: 100px;
    background-size: cover;
}

<div class="img" style="background-image:url('http://i.imgur.com/tI5jq2c.jpg');"></div>
<div class="img" style="background-image:url('http://i.imgur.com/37w80TG.jpg');"></div>
<div class="img" style="background-image:url('http://i.imgur.com/B1MCOtx.jpg');"></div>

You can use the object-fit property to size the img elements:

• cover stretches or shrinks the image proportionally to fill the container. The image is cropped horizontally -or- vertically if necessary.
• contain stretches or shrinks the image proportionally to fit inside the container.
• scale-down shrinks the image proportionally to fit inside the container.

.example {
  margin: 1em 0;
  text-align: center;
}

.example img {
  width: 30vw;
  height: 30vw;
}

.example-cover img {
  object-fit: cover;
}

.example-contain img {
  object-fit: contain;
}

<div class="example example-cover">
  <img src="https://i.stack.imgur.com/B0EAo.png">
  <img src="https://i.stack.imgur.com/iYkNH.png">
  <img src="https://i.stack.imgur.com/gne9N.png">
</div>

<div class="example example-contain">
  <img src="https://i.stack.imgur.com/B0EAo.png">
  <img src="https://i.stack.imgur.com/iYkNH.png">
  <img src="https://i.stack.imgur.com/gne9N.png">
</div>

In the above example: red is landscape, green is portrait and blue is square image. The checkered pattern consists of 16x16px squares.

Simplest way - This will keep the image size as it is and fill the other area with space, this way all the images will take same specified space regardless of the image size without stretching

.img{
   width:100px;
   height:100px;

/*Scale down will take the necessary specified space that is 100px x 100px without stretching the image*/
    object-fit:scale-down;

}

can i just throw in that if you distort your images too much, ie take them out of a ratio, they may not look right, - a tiny amount is fine, but one way to do this is put the images inside a 'container' and set the container to the 100 x 100, then set your image to overflow none, and set the smallest width to the maximum width of the container, this will crop a bit of your image though,

for example

<h4>Products</h4>
<ul class="products">
    <li class="crop">
        <img src="ipod.jpg" alt="iPod" />
    </li>
</ul>



.crop {
 height: 300px;
 width: 400px;
 overflow: hidden;
}
.crop img {
 height: auto;
 width: 400px;
}

This way the image will stay the size of its container, but will resize without breaking constraints