diff --git a/sphinx/themes/basic/static/basic.css_t b/sphinx/themes/basic/static/basic.css_t
index 83af2303a..d54e7f4ec 100644
--- a/sphinx/themes/basic/static/basic.css_t
+++ b/sphinx/themes/basic/static/basic.css_t
@@ -89,6 +89,7 @@ div.sphinxsidebar #searchbox input[type="submit"] {
 
 img {
     border: 0;
+    max-width: 100%;
 }
 
 /* -- search page ----------------------------------------------------------- */
diff --git a/sphinx/themes/epub/static/epub.css b/sphinx/themes/epub/static/epub.css
index 5e5f07c13..3f4664f6d 100644
--- a/sphinx/themes/epub/static/epub.css
+++ b/sphinx/themes/epub/static/epub.css
@@ -92,6 +92,7 @@ div.sphinxsidebar input {
 
 img {
     border: 0;
+    max-width: 100%;
 }
 
 /* -- search page ----------------------------------------------------------- */