diff --git a/bcc.spec b/bcc.spec
index 6eb5854..d0cfa2a 100644
--- a/bcc.spec
+++ b/bcc.spec
@@ -98,7 +98,7 @@ else:
         debug=False,
         bootloader_ignore_signals=False,
         strip=False,
-        upx=True,
+        upx=False,    # UPX can strip icon resources from the PE on Windows
         upx_exclude=[],
         runtime_tmpdir=None,
         console=False,
diff --git a/icons/app.ico b/icons/app.ico
index 62a9f2c..2448f48 100644
Binary files a/icons/app.ico and b/icons/app.ico differ
diff --git a/scripts/build_icons.py b/scripts/build_icons.py
index b62496c..7d4923d 100644
--- a/scripts/build_icons.py
+++ b/scripts/build_icons.py
@@ -32,22 +32,79 @@ def build_ico():
         print("Pillow not installed — skipping .ico generation (pip install pillow)")
         return
 
-    sizes = [16, 32, 48, 64, 128, 256]
-    imgs = []
+    import io, struct
+
+    sizes = [16, 24, 32, 48, 64, 128, 256]
+    images: list[tuple[int, object]] = []
     for s in sizes:
         p = SRC / f"icon-{s}.png"
         if not p.exists():
             print(f"  Missing {p.name}, skipping")
             continue
-        imgs.append(Image.open(p).convert("RGBA"))
+        images.append((s, Image.open(p).convert("RGBA")))
 
-    if not imgs:
+    if not images:
         print("  No source PNGs found — cannot build .ico")
         return
 
+    # Write the ICO manually so that entries ≤ 128 px use uncompressed BMP DIB
+    # and the 256 px entry uses PNG.  Pillow's ICO saver stores all sizes as
+    # PNG-compressed ("Vista icon" format), which PyInstaller's Windows
+    # resource-updater cannot embed — it silently falls back to its default icon.
+
+    def bmp_dib(img: object) -> bytes:
+        """Return a BMP DIB (BITMAPINFOHEADER + BGRA rows + AND mask)."""
+        w, h = img.size
+        # biHeight is doubled: top half = XOR mask (color), bottom = AND mask
+        hdr = struct.pack(
+            "<IiiHHIIiiII",
+            40,     # biSize
+            w,
+            h * 2,  # biHeight (doubled per ICO convention)
+            1,      # biPlanes
+            32,     # biBitCount
+            0,      # biCompression (BI_RGB)
+            0,      # biSizeImage (0 ok for BI_RGB)
+            0, 0, 0, 0,
+        )
+        pix = img.load()
+        rows = bytearray()
+        for y in range(h - 1, -1, -1):   # bottom-up
+            for x in range(w):
+                r, g, b, a = pix[x, y]
+                rows += bytes([b, g, r, a])
+        # AND mask: 1 bit/pixel, rows padded to 32-bit boundary, all 0 (use alpha)
+        mask_row = ((w + 31) // 32) * 4
+        and_mask = bytes(mask_row * h)
+        return hdr + bytes(rows) + and_mask
+
+    def png_bytes(img: object) -> bytes:
+        buf = io.BytesIO()
+        img.save(buf, format="PNG")
+        return buf.getvalue()
+
+    blobs: list[tuple[int, bytes]] = []
+    for s, img in images:
+        blobs.append((s, bmp_dib(img) if s < 256 else png_bytes(img)))
+
+    n = len(blobs)
+    dir_offset = 6 + n * 16   # ICONDIR (6) + n × ICONDIRENTRY (16)
+
+    out = bytearray()
+    out += struct.pack("<HHH", 0, 1, n)   # ICONDIR
+
+    cur = dir_offset
+    for s, blob in blobs:
+        w = h = s % 256    # 256 is stored as 0 in the byte field
+        out += struct.pack("<BBBBHHII", w, h, 0, 0, 1, 32, len(blob), cur)
+        cur += len(blob)
+
+    for _, blob in blobs:
+        out += blob
+
     dest = OUT / "app.ico"
-    imgs[-1].save(dest, format="ICO", append_images=imgs[:-1])
-    print(f"  Generated {dest.relative_to(ROOT)}  ({dest.stat().st_size // 1024 + 1} KB)")
+    dest.write_bytes(out)
+    print(f"  Generated {dest.relative_to(ROOT)}  ({len(out) // 1024 + 1} KB)")
 
 
 def build_icns():