Well, you can say in a simpler way that you work with 2D images and therefore you need to provide this information to the metric. 

Even better in a more "pythonic" way I would write that:
dim = static.ndim
metric = SSDMetric(dim, step_type = 'gauss_newton')

Anyway, my point here is that the term "domain" that you use here will not mean much to most people especially those who are not much into math.