You will notice that the picture drawing region is bordered on the left and above with three banks of small squares, coloured red, green and blue. With a little practice, these give an extremely powerful way of manipulating the image – particularly if it involves simple geometric shapes.

Manipulating the image
The red squares delete the corresponding row or column and move remaining pixels up or left respectively. The green squares insert a row or column in the default colour (grey), and the blue squares replicate the row or column. Both the green and blue squares push the remaining cells down or right, with the last row or column falling off the edge.

Using these controls, it is very easy to centre an image (just add or remove rows or columns) or make a shape fatter or thinner.