docs(readme): update gutter image in README files

Replace the gutter.png image with gutter.jpg across all README files
(English, Simplified Chinese, and Traditional Chinese) for improved
compatibility and reduced file size.

README-zh_CN.md

@@ -131,7 +131,7 @@
 
 在边栏列中显示标题装饰，而不是内联在文本中。适用于实时阅览和源码模式。详细可参考 [#24](https://github.com/dragonish/obsidian-heading-decorator/issues/24)。预览效果：
 
-![gutter](images/gutter.png)
+![gutter](images/gutter.jpg)
 
 ### 阅读视图的其他设置
 

README-zh_TW.md

@@ -131,7 +131,7 @@
 
 在邊欄列中顯示標題裝飾，而不是內嵌在文字中。適用於實際預覽和原始碼模式。詳細可參考 [#24](https://github.com/dragonish/obsidian-heading-decorator/issues/24)。預覽效果：
 
-![gutter](images/gutter.png)
+![gutter](images/gutter.jpg)
 
 ### 閱讀的其他設定
 

README.md

@@ -133,7 +133,7 @@ For the *Ignore the single heading at the top-level* setting, if the top-level h
 
 Render heading decorations in a gutter column instead of inline with the text. Applies to both live preview and source mode. For details, see [#24](https://github.com/dragonish/obsidian-heading-decorator/issues/24). Preview effect:
 
-![gutter](images/gutter.png)
+![gutter](images/gutter.jpg)
 
 ### Other settings for reading view