After I thought about it, I suppose that x1 would always equal x3, etc. so the margin idea makes more sense.
We gotta make sure that the image tiles are repeated along the axis, not so much stretched. the gradient effects as most designers like to use for such objects would not turn out as expected if the image were simply stretched.
I also think the word “slice” is applicable, because designers already use the slice tool in Illustrator, Photoshop and Fireworks to make this effect by other more complicated methods.