diff --git a/convert_png.py b/convert_png.py
index a52963b..e1848a4 100644
--- a/convert_png.py
+++ b/convert_png.py
@@ -1,4 +1,15 @@
-# Convert png image to 256 palette image for Pokitto.
+# Convert png image to 256 palette image for Pokitto. Usage:
+#
+#   python convert_png.py input_image.png palette_16x16_image.png
+#
+# The script will read the input PNG image (must be in RGB format, NOT
+# indexed), match each pixel to the closest color in the palette and output
+# the image to stdout as an array of bytes, by columns. It's best if the image
+# is made only of color of the palette as the closest match can sometimes look
+# different from what you imagined.
+#
+# The script will also create an image (out.png) of the converted image, so
+# that you can check how it looks.
 #
 # author: Miloslav Ciz
 # license: CC0