Short description of error
It seems background image offset Y doesn't work as expected.
Exact steps for others to reproduce the error
Create a new rectangular image, for example 400x600 pixels
Add it to view 3d as background image
Set size to 4, so width will represents 4 units and height 6 units
Set Y offset to 2, the image will be shifted to 3 because aspect ratio is 1.5. To get a shift of 2 units we need to set Y offset to 1.3333