Add shadow effect option for images

_posts/2019-08-08-text-and-typography.md

@@ -57,7 +57,6 @@ Fluttering and dancing in the breeze.
 
 - [ ] TODO
 - [x] Completed
-- Hold on
 - [ ] Defeat COVID-19
   - [x] Vaccine production
   - [ ] Economic recovery
@@ -108,6 +107,11 @@ _Full screen width and center alignment_
 ![Desktop View](https://cdn.jsdelivr.net/gh/cotes2020/chirpy-images/posts/20190808/mockup.png){: width="400"}
 _400px image width_
 
+- Shadow
+
+![Shadow Avatar](https://cdn.jsdelivr.net/gh/cotes2020/chirpy-images/posts/20190808/window.png){: .shadow width="90%" }
+_shadow effect (visible in light mode)_
+
 <br>
 
 - Left aligned

_posts/2019-08-08-write-a-new-post.md

@@ -135,7 +135,16 @@ By default, the image is centered, but you can specify the position by using one
   ![Desktop View](/assets/img/sample/mockup.png){: .right}
   ```
 
-**Limitation**: Once you specify the position of an image, it is forbidden to add the image caption.
+> **Limitation**: Once you specify the position of an image, it is forbidden to add the image caption.
+
+### Image shadow
+
+The screenshots of the program window can be considered to show the shadow effect, and the shadow will be visible in the `light` mode:
+
+```markdown
+![Desktop View](/assets/img/sample/mockup.png){: .shadow}
+```
+
 
 ### CDN URL