Added text alternatives to the images and changed the icon used in the "Free & OpenSource" section

Author: GarkGarcia

images/gpl.svg

@@ -32,7 +32,7 @@ <h1><a href="#">Mathics</a></h1>
             </li>
 					</ul>
 				</div>
-				<img src="images/logo-heptatom.svg" alt=""  />
+				<img src="images/logo-heptatom.svg" alt="Mathics logo"  />
 			</header>
 
 		<!-- Description -->
@@ -53,7 +53,7 @@ <h2>
 				<div class="inner alt">
 					<section class="spotlight">
 						<div class="image">
-              <img src="images/open-source.svg" alt="" />
+              <img src="images/gpl.svg" alt="GPLv3 logo" />
             </div>
 						<div class="content">
 							<h3>Free & OpenSource</h3>
@@ -79,7 +79,9 @@ <h3>Python integration</h3>
 						</div>
 					</section>
 					<section class="spotlight">
-						<div class="image"><img src="images/cli.png" alt="" /></div>
+						<div class="image">
+              <img src="images/cli.png" alt="mathicsscript screenshot" />
+            </div>
 						<div class="content">
 							<h3>Flexible clients</h3>
 							<p>Mathics is built to work on multiple clients. The main ones
@@ -249,19 +251,22 @@ <h2>Screenshots</h2>
             <dt>mathicsscript</dt>
             <dd>
               <div class="image">
-                <img src="images/mathicsscript1.gif"></img>
+                <img src="images/mathicsscript1.gif" 
+                     alt="mathicsscript screenshot" />
               </div>
             </dd>
             <dt>mathicsscript</dt>
             <dd>
               <div class="image">
-                <img src="images/mathicsscript2.gif"></img>
+                <img src="images/mathicsscript2.gif"
+                     alt="mathicsscript screenshot" />
               </div>
             </dd>
             <dt>mathicsserver</dt>
             <dd>
               <div class="image">
-                <img src="images/mathicsserver.png"></img>
+                <img src="images/mathicsserver.png"
+                     alt="Mathics-Django screenshot" />
               </div>
             </dd>
 				</header>