You can use "X/Y Offset" to align a background image to the axis, they just need to have the same value as "Size". This patch increases the offset from -250 to 250 (old values were -20 to 20) so that you can also align images with a "Size" bigger than 20.

Future improvements:
As the "X/Y Offset" should always have the same values as "Size" a makro for this value would make sense.