Height of the picture in pixels must be divisible (multiple) 10, when you create numbers.png file with 10 figures. For example 180 pixels, 190 pixels, 200 pixels... And vertical distance between same parts of figures (bottoms or tops) must be equal - 18 pixels, 19 pixels, 20 pixels...
Height of months.png in pixels must be divisible 12 with equal distance
between bottom line of letters.
You can adjust position of all your numbers and months in calendar layout grid by change parameter align="..."
. See Help > User Guide > Skin Format > Item Positions.
Sorry for my bad English. I'm Ukrainian.