A custom site logo is supported in Business Identity; it is automatically constrained to 48 pixels by 48 pixels. If you would like to use a high quality logo for Retina devices then simply upload a 96 pixel by 96 pixel logo and it will be reduced to 48 pixels by 48 pixels next to your website title.

Alternate Method

If 48 pixels by 48 pixels is too small for you, then you can use an alternate method for adding a site logo into your website with Custom CSS. Using this method will require that you do not hide your site title. If, for example, you would like to use a logo that is 200 pixels wide by 50 pixels tall then you could use the following CSS:

.site-title img {
	display: none;
.site-title a span:first-child {
	background: url( 'https://streamsideventures.files.wordpress.com/2014/08/sample-logo.png' );
	display: inline-block;
	height: 50px;
	width: 200px;
	text-indent: -9999em;
	background-repeat: no-repeat;
	background-position: center center;
	-webkit-background-size: cover;
	-moz-background-size: cover;
	-o-background-size: cover;
	background-size: cover;

You will need to remember to change the url, height, and width values in the above sample to fit your own needs.