iframes don't work in normal markdown

Author: ImLucasBrown

sub_graph_tutorial/README.md

@@ -1,7 +1,8 @@
 # Sub-graphs
 The content in this directory relates to the "Using sub-graphs" tutorial.
 
-<iframe width="560" height="315" src="https://www.youtube.com/embed/xpVkEMWyTao" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+
+[![Using sub-graphs](https://img.youtube.com/vi/xpVkEMWyTao/0.jpg)](https://www.youtube.com/watch?v=xpVkEMWyTao)
 - Referencing builtin layers
 - Calling sub-graphs
 - Getting and using returns from sub-graphs