diff --git a/backend.py b/backend.py
index 401a93e..08ae268 100644
--- a/backend.py
+++ b/backend.py
@@ -2,6 +2,7 @@ from PIL import Image
 from io import BytesIO
 from mimetypes import types_map as mimetypes
 from zipfile import ZipFile
+from zlib import crc32
 import os.path
 import sqlite3
 import werkzeug.exceptions as exceptions
@@ -20,23 +21,28 @@ def dict_factory(cursor, row):
 
 
 def generate_thumbnail(filepath):
-    if filepath in thumbnail_cache:
-        thumbnail = BytesIO()
-        thumbnail.write(thumbnail_cache[filepath])
-    else:
+    if filepath not in thumbnail_cache:
         image = Image.open(filepath)
         image.thumbnail((512, 512))
         thumbnail = BytesIO()
         image.save(thumbnail, 'webp')
         thumbnail.seek(0)
-        thumbnail_cache[filepath] = thumbnail.read()
 
-    thumbnail.seek(0)
+        data = thumbnail.read()
+        etag = str(crc32(data))
 
-    return {
-        'data': thumbnail,
-        'mimetype': 'image/webp'
-    }
+        thumbnail_cache[filepath] = {
+            'data_raw': data,
+            'etag': etag,
+            'mimetype': 'image/webp'
+        }
+
+    thumbnail = thumbnail_cache[filepath]
+    thumbnail['data'] = BytesIO()
+    thumbnail['data'].write(thumbnail['data_raw'])
+    thumbnail['data'].seek(0)
+
+    return thumbnail
 
 
 class CalibreDB:
diff --git a/mangareader.py b/mangareader.py
index 7a18512..49e6235 100755
--- a/mangareader.py
+++ b/mangareader.py
@@ -3,7 +3,6 @@ from backend import CalibreDB
 from flask import Flask, jsonify, send_from_directory, send_file, render_template, request
 from flask_cors import CORS
 from werkzeug.routing import BaseConverter
-from zlib import crc32
 import os
 
 
@@ -11,10 +10,7 @@ def send_from_cwd(filename):
     return send_from_directory(os.getcwd(), filename)
 
 
-def send_file_with_etag(fp, etag=None, **kwargs):
-    if etag is None:
-        etag = str(crc32(fp.read()))
-
+def send_file_with_etag(fp, etag, **kwargs):
     if request.if_none_match and etag in request.if_none_match:
         return '', 304
 
@@ -24,6 +20,10 @@ def send_file_with_etag(fp, etag=None, **kwargs):
     return response
 
 
+def send_image(image):
+    return send_file_with_etag(image['data'], image['etag'], mimetype=image['mimetype'])
+
+
 app = Flask(__name__, static_folder='frontend/dist/static', template_folder='frontend/dist')
 app.config['SEND_FILE_MAX_AGE_DEFAULT'] = 604800  # 1 week
 CORS(app)
@@ -51,7 +51,7 @@ def get_series_cover(series_id):
 @app.route('/api/series/<int:series_id>/cover/thumbnail')
 def get_series_cover_thumbnail(series_id):
     thumbnail = db.get_series_cover_thumbnail(series_id)
-    return send_file_with_etag(thumbnail['data'], mimetype=thumbnail['mimetype'])
+    return send_image(thumbnail)
 
 
 @app.route('/api/series/<int:series_id>')
@@ -67,7 +67,7 @@ def get_volume_cover(volume_id):
 @app.route('/api/volume/<int:volume_id>/cover/thumbnail')
 def get_volume_cover_thumbnail(volume_id):
     thumbnail = db.get_volume_cover_thumbnail(volume_id)
-    return send_file_with_etag(thumbnail['data'], mimetype=thumbnail['mimetype'])
+    return send_image(thumbnail)
 
 
 @app.route('/api/volume/<int:volume_id>')
@@ -78,8 +78,7 @@ def get_volume_info(volume_id):
 @app.route('/api/volume/<int:volume_id>/page/<int:page_number>')
 def get_volume_page(volume_id, page_number):
     page = db.get_volume_page(volume_id, page_number)
-    response = send_file_with_etag(page['data'], etag=page['etag'], mimetype=page['mimetype'])
-    return response
+    return send_image(page)
 
 
 if __name__ == '__main__':