If you only have one image then its not a sprite as such so the question doesn't really make sense?
A sprite becomes a 'sprite' when more than one image is present (e.g. two states of a rollover image are present in the sprite although the sprite could contain as many as a 100 images or more as in this sprite of country flags where there are more than 300 images combined in one.
As I said before if you only have 2 images then it doesn't matter. If you have multiple images in the sprite that are the same size then it doesn't matter there either.
On the other hand if you have irregular shaped images you may want to stack them in a best fit scenario and that will depend on the width and height of individual images and in many cases they may be stacked horizontally and vertically like the flag image I showed but with different sized images inside.
In the end it doesn't matter as long as you make best use of space.
Here's a sprite from google and they have arranged them vertically because they are all small icons.