Smaller screenshot preview and center them

README.rst

@@ -14,27 +14,17 @@ See screenshots below.
 
 Theme ``default``:
 
-.. image:: https://github.com/ymattw/ydiff/blob/master/img/side-by-side-default.png
-   :alt: side by side, theme 'default'
-   :align: center
-   :height: 600 px
-
-.. image:: https://github.com/ymattw/ydiff/blob/master/img/unified-default.png
-   :alt: unified, theme 'default'
-   :align: center
-   :height: 600 px
+<p align="center">
+  <img src="https://github.com/ymattw/ydiff/blob/master/img/side-by-side-default.png" alt="side by side, theme 'default'" height="300">
+  <img src="https://github.com/ymattw/ydiff/blob/master/img/unified-default.png" alt="unified, theme 'default'" height="300">
+</p>
 
 Theme ``highlight``:
 
-.. image:: https://github.com/ymattw/ydiff/blob/master/img/side-by-side-highlight.png
-   :alt: side by side, theme 'highlight'
-   :align: center
-   :height: 600 px
-
-.. image:: https://github.com/ymattw/ydiff/blob/master/img/unified-highlight.png
-   :alt: unified, theme 'highlight'
-   :align: center
-   :height: 600 px
+<p align="center">
+  <img src="https://github.com/ymattw/ydiff/blob/master/img/side-by-side-highlight.png" alt="side by side, theme 'highlight'" height="300">
+  <img src="https://github.com/ymattw/ydiff/blob/master/img/unified-highlight.png" alt="unified, theme 'highlight'" height="300">
+</p>
 
 Ydiff only supports diff in `Unified Format`_. This is default in most version
 control system except Perforce, which needs an environment variable