How do I edit Walden's Newsletter Row Background Image?

First, navigate to Appearance > Editor and scroll down to subtitle "00 - THEME BUGS." Go to the subsection "NEWSLETTER ROW BUGS" replace the URL of the image (http://extra.timberridgemedia.com/wp-content/uploads/2016/02/unplash17.jpg) with your own URL. Here's how it looks:

 /* NEWSLETTER ROW BUGS */      

#newsletter {<br>	padding: 30px 0px; }<br>#newsletter .et_pb_row {<br>        padding: 70px 40px;         <br>	background-image: url("http://extra.timberridgemedia.com/wp-content/uploads/2016/02/unplash17.jpg");<br>	background-size: cover; }<br>#newsletter h1 {         <br>	font-size: 32px; }<br>#newsletter .et_pb_button {<br>	margin-top: 10px;         <br>	background:  rgba(255, 145, 96, 0.8);         <br>	border-color:  rgba(255, 145, 96, 0.5); }     
#newsletter .et_pb_button:hover {         <br>	margin-top: 10px;         <br>	background:  #FF9160;         <br>	border-color:  #FF9160; }

That should do the trick!