diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index ef5e192..f6835f7 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -103,3 +103,10 @@ repos:
       - id: gitleaks
         name: run gitleaks
         description: check for secrets with gitleaks
+  - repo: https://github.com/oxipng/oxipng
+    rev: 628e241e23f368097883807fa6e985ccf7c00357 # frozen: v10.1.1
+    hooks:
+      - id: oxipng
+        name: run oxipng
+        description: check PNG files with oxipng
+        args: ['--fix', '-o', '4', '--strip', 'safe', '--alpha']
diff --git a/docs/image/datasets-continents.png b/docs/image/datasets-continents.png
index b31f05a..4f14997 100644
Binary files a/docs/image/datasets-continents.png and b/docs/image/datasets-continents.png differ
diff --git a/docs/image/datasets-schema.png b/docs/image/datasets-schema.png
index 06153fe..fd42a0a 100644
Binary files a/docs/image/datasets-schema.png and b/docs/image/datasets-schema.png differ