improve image docs

Signed-off-by: Jade Abraham <jade.abraham@hpe.com>

modules/packages/Image.chpl

@@ -17,7 +17,7 @@
  * limitations under the License.
  */
 
-/* Provides a way to write arrays of pixels to an output image format.
+/* Provides a way to read/write arrays of pixels to image formats.
 
 For example, the following code creates a 3x3 array of pixels and writes it to
 a BMP file. The array is then scaled by a factor of 2 (creating a 9x9 image)
@@ -39,6 +39,19 @@ and written to a second BMP file.
    writeImage("pixels.bmp", imageType.bmp, arr);
    writeImage("pixels2.bmp", imageType.bmp, scale(arr, 2));
 
+In another example, the following code reads a PNG file, removes all green from
+the image, and writes it back out to a new JPG file.
+
+.. code-block:: chapel
+
+   use Image;
+
+   var arr = readImage("input.png", imageType.png);
+   const fmt = (rgbColor.red, rgbColor.green, rgbColor.blue);
+   var colors = pixelToColor(arr, format=fmt);
+   [x in colors] c(1) = 0;
+   arr = colorToPixel(colors, format=fmt);
+   writeImage("output.jpg", imageType.jpg, arr);
 
 */
 @unstable("Image is unstable")