mysite-_config-config.yml

@@ -8,4 +8,8 @@ After:
 # See http://doc.silverstripe.org/framework/en/topics/configuration
 # Caution: Indentation through two spaces, not tabs
 SSViewer:
-  theme: 'sam'
\ No newline at end of file
+  theme: 'sam'
+
+SiteConfig:
+  extensions:
+   - CustomConfig

mysite-code-CustomConfig.php

<?php

class CustomConfig extends DataExtension
{

	private static $has_one = array(
		"Logo" => "Image",
	);

	function updateCMSFields(FieldList $fields) {

		$fields->addFieldToTab('Root.Main', 
			new UploadField('Logo')
		);

	}

}

themes-sam-templates-Includes-Header.ss

@@ -2,7 +2,15 @@
 	<div class="inner">
 		<div class="unit size4of4 lastUnit">
 			<a href="$BaseHref" class="brand" rel="home">
-				<h1>$SiteConfig.Title</h1>
+				<h1>
+								<% if SiteConfig.Logo %>
+				<img 
+					style="float: left; margin-right: 10px" 
+					src="$SiteConfig.Logo.SetWidth(100).URL">
+				<% end_if %>
+
+					$SiteConfig.Title
+				</h1>
 				<% if $SiteConfig.Tagline %>
 				<p>$SiteConfig.Tagline</p>
 				<% end_if %>