Update README.md

Author: NikolajBjorner

README.md

@@ -12,7 +12,8 @@ Z3 can be built using [Visual Studio][1], a [Makefile][2] or using [CMake][3]. I
 
 See the [release notes](RELEASE_NOTES.md) for notes on various stable releases of Z3.
 
-[Try it online](https://microsoft.github.io/z3guide/)
+[![Try the online Z3 Guide](z3guide.jpeg)](https://microsoft.github.io/z3guide/)
+
 
 
 ## Build status