basics: Adding image with example of requests

tutos/7.1/manual/basics.wiki

@@ -577,6 +577,17 @@ let%client c : int = 3 + ~%x
 
 <<div class="focused"|
 ===How it works===
+
+The following picture shows two examples of requests:
+* First, the browser ask for a new page, and the server generates the page
+* then the user clicks on a link in the page, and the page is generated by the client-side program (because the service is registered on both sides). In this example, while generating the page, the client does a rpc to the server.
+
+In both cases (first request or rpc), the server returns the expected value, but also the value of injections and an order for the client-side program to execute the client-values met during te server-side computation.
+
+@@class="centered"@@{{@@class="img-col-width-400"@@files/tutorial/client-server-req.svg|Example of requests}}
+
+Tip: You can avoid waiting for the rpc to return by using a spinner from Ocsigen Toolkit. Thus, the client-side generated page will be displayed without delay.
+
 Regardless of the construction used and their combination, there is
 only one communication from server to client, when the Web page is
 sent. This is due to the fact that client fragments are not executed