diff --git a/clip-thumbnailer.py b/clip-thumbnailer.py
index 0261937..9c874da 100755
--- a/clip-thumbnailer.py
+++ b/clip-thumbnailer.py
@@ -1,4 +1,5 @@
 #!/usr/bin/env python
+import io
 import os
 import sqlite3
 import sys
@@ -155,10 +156,31 @@ def _best_image(data: bytes) -> Optional[Tuple[int, int]]:
 def _parse_args(argv):
     # Accept: [size, input, output] or [input, output]
     if len(argv) == 3:
-        return argv[1], argv[2]
+        return None, argv[1], argv[2]
     if len(argv) == 4:
-        return argv[2], argv[3]
-    return None, None
+        return argv[1], argv[2], argv[3]
+    return None, None, None
+
+
+def _write_preview(preview: bytes, out_path: str, size: Optional[int]) -> bool:
+    try:
+        from PIL import Image
+    except Exception:
+        Image = None
+
+    if Image is None:
+        if preview.startswith(b"\x89PNG\r\n\x1a\n") and size is None:
+            with open(out_path, "wb") as f:
+                f.write(preview)
+            return True
+        return False
+
+    with Image.open(io.BytesIO(preview)) as im:
+        im.load()
+        if size:
+            im.thumbnail((size, size), Image.Resampling.LANCZOS)
+        im.save(out_path, format="PNG")
+    return True
 
 
 def main() -> int:
@@ -204,14 +226,17 @@ def main() -> int:
                 exit_code = 1
                 continue
             try:
-                with open(out_path, "wb") as f:
-                    f.write(preview)
+                if not _write_preview(preview, out_path, None):
+                    sys.stderr.write(
+                        f"failed to write preview for {in_path} (missing Pillow or unsupported format)\n"
+                    )
+                    exit_code = 1
             except OSError as exc:
                 sys.stderr.write(f"failed to write {out_path}: {exc}\n")
                 exit_code = 1
         return exit_code
 
-    in_path, out_path = _parse_args(sys.argv)
+    size_str, in_path, out_path = _parse_args(sys.argv)
     if not in_path or not out_path:
         sys.stderr.write(
             "usage: clip-thumbnailer [size] INPUT OUTPUT | clip-extract-preview INPUT OUTPUT | "
@@ -219,6 +244,13 @@ def main() -> int:
         )
         return 2
 
+    size = None
+    if size_str:
+        try:
+            size = max(1, int(size_str))
+        except ValueError:
+            size = None
+
     try:
         with open(in_path, "rb") as f:
             data = f.read()
@@ -229,8 +261,9 @@ def main() -> int:
     preview = _extract_canvas_preview(data)
     if preview is not None:
         try:
-            with open(out_path, "wb") as f:
-                f.write(preview)
+            if not _write_preview(preview, out_path, size):
+                sys.stderr.write("failed to write preview (missing Pillow or unsupported format)\n")
+                return 1
         except OSError as exc:
             sys.stderr.write(f"failed to write {out_path}: {exc}\n")
             return 1
@@ -243,8 +276,9 @@ def main() -> int:
 
     start, end = loc
     try:
-        with open(out_path, "wb") as f:
-            f.write(data[start:end])
+        if not _write_preview(data[start:end], out_path, size):
+            sys.stderr.write("failed to write preview (missing Pillow or unsupported format)\n")
+            return 1
     except OSError as exc:
         sys.stderr.write(f"failed to write {out_path}: {exc}\n")
         return 1
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..bd8c7dc
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,26 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1773371044,
+        "narHash": "sha256-UioNXlVNmX+3QQ0ZjdZkZFq7ctHyObpcvdHm9aZV0/c=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "65feaa4cdcddbc1659abb8796810f29c4531aad8",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
index c2c8903..4cdece9 100644
--- a/flake.nix
+++ b/flake.nix
@@ -12,6 +12,7 @@
       packages = forAllSystems (system:
         let
           pkgs = import nixpkgs { inherit system; };
+          pythonEnv = pkgs.python3.withPackages (ps: [ ps.pillow ]);
         in
         {
           clip-thumbnailer = pkgs.stdenvNoCC.mkDerivation {
@@ -26,6 +27,8 @@
               runHook preInstall
 
               install -Dm755 ${./clip-thumbnailer.py} $out/bin/clip-thumbnailer
+              substituteInPlace $out/bin/clip-thumbnailer --replace \
+                "#!/usr/bin/env python" "#!${pythonEnv}/bin/python3"
               ln -s $out/bin/clip-thumbnailer $out/bin/clip-extract-preview
               install -Dm644 ${./clip.thumbnailer} $out/share/thumbnailers/clip.thumbnailer
               install -Dm644 ${./clip-studio.xml} $out/share/mime/packages/clip-studio.xml