How to scale image to fit the container?

Question

I have an image list, I want images scaled into their containers which have same size. like this:

I created a jsfiddle

<div class="container">
    <div class="row">
        <div class="col-xs-3">
            <a href="#" class="thumbnail">
                <img src="http://exmoorpet.com/wp-content/uploads/2012/08/cat.png">
            </a>
        </div>
        <div class="col-xs-3">
            <a href="#" class="thumbnail">
                <img src="http://www.nose2tail.co.uk/cat-matlock-derbyshire.jpg">
            </a>
        </div>
        <div class="col-xs-3">
            <a href="#" class="thumbnail">
                <img src="http://www.us.onsior.com/images/3_1/cat-3_1-01.png">
            </a>
        </div>
        <div class="col-xs-3">
            <a href="#" class="thumbnail">
                <img src="https://www.petfinder.com/wp-content/uploads/2012/11/155293403-cat-adoption-checklist-632x475-e1354290788940.jpg" >
            </a>
        </div>
    </div>
</div>

How can I do that? And in my example, I defined height: 100px;, this leads to not responsive, if I resize the browser, the div's height remain unchanged. If possible, I want this image list responsive.

Answer 1

Change the height and width to max-height and max-width. The image won't be any bigger than it's parent.

.thumbnail img {
    max-height: 100%;
    max-width: 100%;
}

Updated Fiddle

Answer 2

.thumbnail {
    height: 100px;
    display: table;
}

.thumbnail img {
    height: 100%;
    width: 100%;
    display: table-cell;
    vertical-align: middle;
}

Answer 3

In 2017 you can use flex. Additionally you will get the images centered in the thumbnail container: updated fiddle

.thumbnail {
    height: 300px;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    border: solid 1px blue;
}

.thumbnail img {
    max-width: 100%;
    max-height: 100%;
    display: block;
    margin: 0 auto;
    border: solid 1px red;
}

Answer 4

This should help, at least for your example. There might be some cases in which this will not work. In that case you have to find a js solution.

.thumbnail img {
  height: 100%;
  width: auto;
}