CSS: how to vertically and horizontally align an image?

Question

My page has space for an image that can be, say, a maximum 100x100 image. Users can upload any image dimension and the web application will resize it, maintaining aspect rat

Answer 1

<div class="blog-thumbnail">                
    <img src="img.jpg" alt="img">
</div>

.blog-thumbnail {
    height: 200px;
    margin: 0 auto;
    position: relative;
}
.blog-thumbnail img {
    position: absolute;
    bottom: 0;
    left: 0;
    right: 0;
    top: 0;
    margin: auto;
    max-width: 100%;
    max-height: 100%;
}