diff --git a/assets/lock.png b/assets/lock.png
new file mode 100644
index 0000000000..35d0b9080e
Binary files /dev/null and b/assets/lock.png differ
diff --git a/assets/lock.toi b/assets/lock.toi
new file mode 100644
index 0000000000..e3d557f6f5
Binary files /dev/null and b/assets/lock.toi differ
diff --git a/tools/png2toi b/tools/png2toi
index 23a59b82ec..915b73a796 100755
--- a/tools/png2toi
+++ b/tools/png2toi
@@ -5,11 +5,15 @@ import struct
 import zlib
 
 if len(sys.argv) < 2:
-    print('Usage png2toi image.png')
+    print('Usage png2toi image.png [mode]')
     sys.exit(1)
 
 ifn = sys.argv[1]
 
+gray = False
+if len(sys.argv) >= 3 and sys.argv[2] == 'g':
+    gray = True
+
 if not ifn.endswith('.png'):
     print('Must provide PNG file')
     sys.exit(2)
@@ -18,22 +22,40 @@ im = Image.open(ifn)
 w, h = im.size
 print('Opened  %s ... %d x %d @ %s' % (ifn, w, h, im.mode))
 
-if not im.mode == 'RGB':
-    print('PNG file must use RGB mode')
-    sys.exit(3)
+if not gray:
+    if not im.mode == 'RGB':
+        print('PNG file must use RGB mode')
+        sys.exit(3)
+else:
+    if not im.mode == 'L':
+        print('PNG file must use grayscale mode')
+        sys.exit(3)
+    if w % 2 > 0:
+        print('PNG file must have width divisible by 2')
+        sys.exit(4)
 
 pix = im.load()
 
 ofn = '%s.toi' % ifn[:-4]
 with open(ofn, 'wb') as f:
-    f.write(bytes('TOIa', 'ascii'))
+    if not gray:
+        f.write(bytes('TOIa', 'ascii'))
+    else:
+        f.write(bytes('TOIg', 'ascii'))
     f.write(struct.pack('>HH', w, h))
     data = bytes()
-    for j in range(h):
-        for i in range(w):
-            r, g, b = pix[i, j]
-            c = ((r & 0xF8) << 8) | ((g & 0xFC) << 3) | ((b & 0xF8) >> 3)
-            data += struct.pack('>H', c)
+    if not gray:
+        for j in range(h):
+            for i in range(w):
+                r, g, b = pix[i, j]
+                c = ((r & 0xF8) << 8) | ((g & 0xFC) << 3) | ((b & 0xF8) >> 3)
+                data += struct.pack('>H', c)
+    else:
+        for j in range(h):
+            for i in range(w // 2):
+                l1, l2 = pix[i * 2, j], pix[i * 2 + 1, j]
+                c = (l1 & 0xF0) | (l2 >> 4)
+                data += struct.pack('>B', c)
     z = zlib.compressobj(level=9, wbits=10)
     zdata = z.compress(data) + z.flush()
     zdata = zdata[2:-4] # strip header and checksum