How to change vertical alignment of desktop wallpaper when using Tile

I have a desktop wallpaper which I want to tile, however I'd like to change where it is aligned on the y axis when it is tiled i.e the height from the bottom of my monitor and the margin which it applies to the bottom of the image, at the moment it is is seemingly random.

Is there a preference file I can tinker with?

Thanks

I attach a photograph to show you what I am talking about