diff --git a/src/css/custom.css b/src/css/custom.css
index a72ea3fe..e58dd406 100644
--- a/src/css/custom.css
+++ b/src/css/custom.css
@@ -362,8 +362,13 @@ html[data-theme="dark"] .container img[src$=".png"] { /* Adds a white background
   background-color: #ffffff;
 }
 
+img[class^="img_"] { /* Ensure doc images are always behind overlays and dropdowns. */
+  position: relative;
+  z-index: 1 !important;
+}
+
 .medium-zoom-overlay {
-  z-index: 15;
+  z-index: 1;
   position: fixed;
 }