diff --git a/docs/changelog.rst b/docs/changelog.rst
index c2ff20e..237534f 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -16,6 +16,7 @@ Sigal now requires Python 3.11+.
 - Fix theme URL for photoswipe.
 - Thumbnail can now be set in ``index.md`` either with the original filename or
   with the thumbnail name after format conversion [:issue:`476`].
+- Allow portrait orientation for thumbnails [:issue:`531`].
 
 Version 2.5
 ~~~~~~~~~~~
diff --git a/src/sigal/settings.py b/src/sigal/settings.py
index e81b670..0026d2a 100644
--- a/src/sigal/settings.py
+++ b/src/sigal/settings.py
@@ -194,7 +194,7 @@ def read_settings(filename=None):
                 settings[p] = abspath(normpath(join(settings_path, path)))
                 logger.debug("Rewrite %s : %s -> %s", p, path, settings[p])
 
-    for key in ("img_size", "thumb_size", "video_size"):
+    for key in ("img_size", "video_size"):
         if settings[key]:
             w, h = settings[key]
             if h > w:
diff --git a/tests/test_settings.py b/tests/test_settings.py
index 2ee7872..e08e09f 100644
--- a/tests/test_settings.py
+++ b/tests/test_settings.py
@@ -36,10 +36,14 @@ def test_img_sizes(tmpdir):
     """Test that image size is swaped if needed."""
 
     conf = tmpdir.join("sigal.conf.py")
-    conf.write("thumb_size = (150, 200)")
+    conf.write(
+        "img_size = (600, 800)\n"
+        "thumb_size = (150, 200)"
+    )
 
     settings = read_settings(str(conf))
-    assert settings["thumb_size"] == (200, 150)
+    assert settings["img_size"] == (800, 600)
+    assert settings["thumb_size"] == (150, 200)
 
 
 def test_theme_path(tmpdir):