What software are you using to edit the sprite?
1 - If you're using Photoshop, you can create guides which can remain at the exact point in the image where the game says the edge of the sprite is at, and then as long as you properly match up each sprite to those edges, you should be fine.
2 - If you're using Microsoft Paint, you can use the grid to see exactly how many pixels you are away from the edge of the image and the edge of the other sprites and use that as a measurement.
You can use this as an example for placement in RPG XP:
(Image comes from this link
, which has other information that might be useful to you.)