An answer of the exercise: Slices on a tour of Go

answer_pic.go

package main

import "code.google.com/p/go-tour/pic"

func Pic(dx, dy int) [][]uint8 {
	x := make([]uint8, dx)
	y := make([][]uint8, dy)
	
	for n := range y {
		y[n] = x
		for m := range x {
			y[n][m] = uint8((n+m)/2)
		}
	}
	return y
}

func main() {
	pic.Show(Pic)
}

Here's another answer, mainly pasting to show diff approach on the for statement, using range instead of incrementing.