Update README.md

Author: Yanhang129

README.md

@@ -16,15 +16,18 @@ This repository contains scripts to run the simulation described in [A Polynomia
 ## Visualization
 ### Low-dimensional results
 We get the low-dimensional results:
-<img src="./image/runtime.png" alt="Cover" width="50%"/>
-
+<div align=center>
+<img src="./image/runtime.png" alt="Cover" width="55%"/>
+<\div>
 ### High-dimensional results
 We get the high-dimensional results:
-<img src="./image/high_dimension_result_1.png" alt="Cover" width="50%"/>
-
+<div align=center>
+<img src="./image/high_dimension_result_1.png" alt="Cover" width="60%"/>
+<\div>
 The runtime comparison results are presented as
-<img src="./image/high_dimension_result_2.png" alt="Cover" width="50%"/>
-
+<div align=center>
+<img src="./image/high_dimension_result_2.png" alt="Cover" width="55%"/>
+<\div>
 ## Citations
 
 Please cite the following publications if you make use of the material here.