update readme

Author: yifanfeng97

README.md

@@ -90,10 +90,8 @@ If you find any bugs, please report it to us in [GitHub Issues](https://github.c
 
 You can draw the graph, hypergraph, directed graph, and bipartite graph with DHG's visualization tool. More details see the [Tutorial](https://deephypergraph.readthedocs.io/en/latest/tutorial/vis_structure.html)
 
-<center class="half">
-<img src="docs/source/_static/img/vis_graph.png" width="40%"/><img src="docs/source/_static/img/vis_hypergraph.png" width="40%"/>
-</center>
 
+![Visualization of graph and hypergraph](docs/source/_static/img/readme_graph_hypergraph.png)
 
 ```python
 import matplotlib.pyplot as plt
@@ -108,9 +106,7 @@ hg.draw()
 plt.show()
 ```
 
-<center>
-<img src="docs/source/_static/img/vis_digraph.png" width="300"/><img src="docs/source/_static/img/vis_bigraph.png" width="300"/>
-</center>
+![Visualization of directed graph and bipartite graph](docs/source/_static/img/readme_digraph_bigraph.png)
 
 ```python
 import matplotlib.pyplot as plt