diff --git a/main.py b/main.py
index 8e3b2fa..8b33c0a 100644
--- a/main.py
+++ b/main.py
@@ -448,7 +448,12 @@ def _compile_pdflatex(
 
 
 def _render_png_with_magick(
-    run_dir: Path, pdf_path: Path, run_logger: RunLogger, section_title: str, png_name: str = "diagram.png"
+    run_dir: Path,
+    pdf_path: Path,
+    run_logger: RunLogger,
+    section_title: str,
+    png_name: str = "diagram.png",
+    size: str | None = None,
 ) -> Optional[Path]:
     png_path = run_dir / png_name
 
@@ -464,8 +469,23 @@ def _render_png_with_magick(
         "remove",
         "-alpha",
         "off",
-        str(png_path),
     ]
+
+    # If size is specified, add center-fit resizing (e.g., "1200x630")
+    if size:
+        cmd.extend(
+            [
+                "-resize",
+                size,
+                "-gravity",
+                "center",
+                "-extent",
+                size,
+            ]
+        )
+
+    cmd.append(str(png_path))
+
     code, out = _run_cmd(cmd, cwd=run_dir, timeout_s=5, py_logger=run_logger.py)
     run_logger.section(section_title, out)
     if code == 0 and png_path.exists():
@@ -571,6 +591,19 @@ def _compile_phase(
         if pdf_path
         else None
     )
+    # Render preview.png with OG image size (1200x630, center-fit)
+    preview_path = (
+        _render_png_with_magick(
+            run_dir,
+            pdf_path,
+            run_log,
+            section_title=f"{phase}.magick.preview",
+            png_name="preview.png",
+            size="1200x630",
+        )
+        if pdf_path
+        else None
+    )
     elapsed_ms = int((time.monotonic() - started) * 1000)
 
     _save_compile_state(
diff --git a/templates/edit.html b/templates/edit.html
index 040b573..420f052 100644
--- a/templates/edit.html
+++ b/templates/edit.html
@@ -8,7 +8,7 @@
         <meta property="og:description" content="View and edit the LaTeX/TikZ code for diagram {{ run_id }}." />
         <meta property="og:type" content="website" />
         <meta property="og:url" content="{{ base_path }}/d/{{ run_id }}" />
-        <meta property="og:image" content="{{ base_path }}/d/{{ run_id }}/files/diagram.png" />
+        <meta property="og:image" content="{{ base_path }}/d/{{ run_id }}/files/preview.png" />
         <link rel="preconnect" href="https://fonts.googleapis.com" />
         <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
         <link