add cartridge artwork to docs

docs/index.html

                 margin-top: 10px;
                 margin-bottom: 10px;
             }
+            #cart {
+                max-width: 90%;
+            }
             #controls {
                 margin: 0px auto;
             }
             <p>Press Left or Right on the title screen to show either the black or white highscores. Press Select to show the about screen and build info. In-game press Start to pause and resume. While paused press Select to return to the menu.</p>
             <p>Collect small white spheres to get +5 white score. Collect small black spheres to get +5 black score. The opposite color will reduce your score when collected. Large black holes will attract you and damage your ship when touched. Large white spheres will repel you and replenish your health when touched. Accelerating will reduce your fuel, which will recharge when not accelerating. You can shoot large spheres for +10 points.<p>
             <p>For a more detailed description of the original game check out the <a href="https://gta.fandom.com/wiki/Duality">Duality article on GTA Wiki</a>.</p>
-            <hr>
+        </div>
+        <div class="text">
             <p>Download the <a href="https://xythobuz.github.io/Duality/duality.gb">ROM</a>!</p>
             <p>Get the source code on <a href="https://github.com/xythobuz/Duality">GitHub</a>.</p>
-            <hr>
+        </div>
+        <div class="text">
+            <img id="cart" src="https://xythobuz.github.io/Duality/cartridge.png">
+        </div>
+        <div class="text">
             <p>Emulation supported by <a href="https://emulatorjs.org">EmulatorJS</a>.</p>
         </div>
         <script>