diff --git a/README.md b/README.md
index bae0e5b806edfa34a598b62224ba50a4d6151b4c..e7c041761033072eeffa311e4da9d630955b223f 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,7 @@ nginx or another webserver to ensure SSL encryption (see below for instructions)
 
 ```
 sudo docker run -p 8080:5000 \
+--env EXTERNAL_HOST=<YOUR_EXTERNAL_DOMAIN_OR_ENDPOINT> \
 -v /home/<YOURUSER>/slide-index/img_cache:/app/img_cache \
 -v /home/<YOURUSER>/slide-index/index:/app/index \
 -v /home/<YOURUSER>/slide-index/slides:/app/slides \
diff --git a/app.py b/app.py
index 71b56c756375f3e507bc67248324dd3cc4a59ac0..2746bc2c59c88b1bbc70204006c0d76d93cfc94b 100644
--- a/app.py
+++ b/app.py
@@ -1,5 +1,5 @@
 import os
-import sys
+import socket
 from pathlib import Path
 
 import pdfplumber
@@ -57,7 +57,7 @@ def query():
         img_name = result["path"][7:] + "_" + str(result["page"]) + ".jpg"
         return jsonify({
             "type": "image",
-            "path": "slide/" + img_name
+            "path": os.getenv('EXTERNAL_HOST', '<PLEASE_SET_EXTERNAL_HOST_ENV_VAR>') + "/slide/" + img_name
         })
     except IndexError:
         return jsonify({