diff --git a/src/web/stepper/src/SubstVisualizer.tsx b/src/web/stepper/src/SubstVisualizer.tsx
index 8b81e24..694f9c5 100644
--- a/src/web/stepper/src/SubstVisualizer.tsx
+++ b/src/web/stepper/src/SubstVisualizer.tsx
@@ -397,6 +397,23 @@ function ProfileHoverTextPopover({ text }: { text: string }) {
   );
 }
 
+/** Popover body for an `image` part: the inline opaque-value thumbnail, enlarged to be legible. */
+function EnlargedThumbnailPopover({ src, alt }: { src: string; alt?: string }) {
+  return (
+    <div className={classNames("stepper-popover", Classes.DARK)}>
+      <div className="stepper-display">
+        {alt ? (
+          <>
+            <Icon icon="media" />
+            <span>{` ${alt}`}</span>
+          </>
+        ) : null}
+        <img className="stepper-opaque-thumbnail-large" src={src} alt={alt ?? ""} />
+      </div>
+    </div>
+  );
+}
+
 /**
  * renderNode renders a serialized Stepper AST node to a React ReactNode.
  */
@@ -860,14 +877,23 @@ function renderNode(
         const src = readNodeProp(node, part.image);
         if (typeof src !== "string" || !src.startsWith("data:")) return null;
         const alt = part.altProp === undefined ? undefined : readNodeProp(node, part.altProp);
+        const altText = alt == null ? "" : String(alt);
         return (
-          <img
+          <Popover
             key={key}
-            className={classNames("stepper-opaque-thumbnail", cls(part.cls))}
-            src={src}
-            alt={alt == null ? "" : String(alt)}
-            title={alt == null ? undefined : String(alt)}
-          />
+            interactionKind="hover"
+            placement="bottom"
+            usePortal={popoverDepth === 0}
+            lazy
+            popoverClassName="stepper-popover"
+            content={<EnlargedThumbnailPopover src={src} alt={altText} />}
+          >
+            <img
+              className={classNames("stepper-opaque-thumbnail", cls(part.cls))}
+              src={src}
+              alt={altText}
+            />
+          </Popover>
         );
       }
       return null;
diff --git a/src/web/stepper/src/styles.ts b/src/web/stepper/src/styles.ts
index 801e73f..52f495e 100644
--- a/src/web/stepper/src/styles.ts
+++ b/src/web/stepper/src/styles.ts
@@ -62,6 +62,19 @@ const STEPPER_CSS = `
   border-radius: 3px;
 }
 
+/* The same thumbnail enlarged, shown in the hover popover so an opaque value (e.g. a rune) is
+ * actually legible while the inline copy stays at text height. A fixed width (not max-width) forces
+ * a low-resolution thumbnail to scale up rather than render at its small intrinsic size. */
+.sa-substituter .stepper-opaque-thumbnail-large,
+.stepper-popover .stepper-opaque-thumbnail-large {
+  display: block;
+  width: min(200px, 50vw);
+  height: auto;
+  margin-top: 6px;
+  border-radius: 3px;
+  image-rendering: auto;
+}
+
 .sa-substituter .stepper-mu-term,
 .stepper-popover .stepper-mu-term {
   font-weight: bold;