Add touchguitar image to Karplus Strong documentation

gui/index.html

@@ -1438,6 +1438,7 @@ The actual packets are taken
 	<div class=tooltipinfo>
 	<p>Synthesize a plucked string sound, such as a guitar string.
 		</p>
+	<p align=center><img src="img/touchguitar.jpg"></p>
 	</div>
 	<h3>Audio Connections</h3>
 	<table class=doc align=center cellpadding=3>