diff --git a/README.md b/README.md
index 6d788ac2c8..7f42f86297 100644
--- a/README.md
+++ b/README.md
@@ -131,6 +131,11 @@ Environment variables (will overwrite other server configs)
 | HMD_GOOGLE_CLIENTID | no example | Google API client id |
 | HMD_GOOGLE_CLIENTSECRET | no example | Google API client secret |
 | HMD_IMGUR_CLIENTID | no example | Imgur API client id |
+| HMD_IMAGE_UPLOAD_TYPE | `imgur`, `s3` or `filesystem` | Where to upload image. For S3, see our [S3 Image Upload Guide](docs/guides/s3-image-upload.md) |
+| HMD_S3_ACCESS_KEY_ID | no example | AWS access key id |
+| HMD_S3_SECRET_ACCESS_KEY | no example | AWS secret key |
+| HMD_S3_REGION | `ap-northeast-1` | AWS S3 region |
+| HMD_S3_BUCKET | no example | AWS S3 bucket name |
 
 Server settings `config.json`
 ---
@@ -166,6 +171,8 @@ Server settings `config.json`
 | heartbeatinterval | `5000` | socket.io heartbeat interval |
 | heartbeattimeout | `10000` | socket.io heartbeat timeout |
 | documentmaxlength | `100000` | note max length |
+| imageUploadType | `imgur`(default), `s3` or `filesystem` | Where to upload image
+| s3 | `{ "accessKeyId": "YOUR_S3_ACCESS_KEY_ID", "secretAccessKey": "YOUR_S3_ACCESS_KEY", "region": "YOUR_S3_REGION", "bucket": "YOUR_S3_BUCKET_NAME" }` | When `imageUploadType` be setted to `s3`, you would also need to setup this key, check our [S3 Image Upload Guide](docs/guides/s3-image-upload.md) |
 
 Third-party integration api key settings
 ---
diff --git a/docs/guides/s3-image-upload.md b/docs/guides/s3-image-upload.md
index c92e91f848..3d6259b9bc 100644
--- a/docs/guides/s3-image-upload.md
+++ b/docs/guides/s3-image-upload.md
@@ -57,6 +57,25 @@
     }
     ```
 
+8. Edit `config.json` and set following keys:
+
+    ```javascript
+    {
+        "production": {
+            ...
+            "imageUploadType": "s3",
+            "s3": {
+                "accessKeyId": "YOUR_S3_ACCESS_KEY_ID",
+                "secretAccessKey": "YOUR_S3_ACCESS_KEY",
+                "region": "YOUR_S3_REGION", // example: ap-northeast-1
+                "bucket": "YOUR_S3_BUCKET_NAME"
+            }
+        }
+    }
+    ```
+
+9. In additional to edit `config.json` directly, you could also try [environment variable](https://github.com/hackmdio/hackmd#environment-variables-will-overwrite-other-server-configs).
+
 ## Related Tools
 
 * [AWS Policy Generator](http://awspolicygen.s3.amazonaws.com/policygen.html)