Update index.md

Author: mugurm

docs/index.md

@@ -43,7 +43,7 @@ parameters and return types. On the server side, the server implements this
 interface and runs a gRPC server to handle client calls. On the client side,
 the client has a *stub* that provides exactly the same methods as the server.
 
-![gRPC diagram](../img/grpc_concept_diagram_00.png)
+<img src="../img/grpc_concept_diagram_00.png" class="img-responsive" alt="gRPC diagram">
 
 gRPC clients and servers can run and talk to each other in a variety of
 environments - from servers inside Google to your own desktop - and can