Replace Title with Logo

Customizations must be done via child theme.
http://codex.wordpress.org/Child_Themes

For small changes in CSS only, use Custom CSS plugin
http://wordpress.org/plugins/search.php?q=custom+css

This in child theme stylesheet, or Custom CSS plugin, will replace site title text with image. Change image url to your own, do not use image wider than 288px.

.site-title {
	 text-indent: -9999px;
}

.site-title a {
	background: url("http://dummyimage.com/280x75/3104e3/fff.png") no-repeat scroll 0px 0px transparent;
	display: block; margin: 0 auto;
	width: 280px; height: 75px;
}