diff --git a/ncm/file_util.py b/ncm/file_util.py
index 4d647b1..e3dfcc1 100644
--- a/ncm/file_util.py
+++ b/ncm/file_util.py
@@ -14,6 +14,8 @@ def resize_img(file_path, max_size=(640, 640), quality=90):
 
     if img.size[0] > max_size[0] or img.size[1] > max_size[1]:
         img.thumbnail(max_size, Image.ANTIALIAS)
+        if (img.format == 'PNG'):
+            img = img.convert('RGB')
         img.save(file_path, quality=quality)