I didn't find a option near to "put it in the center" like there is on Corel Draw and GIMP. In Aseprite I normally:
- Count how many pixels are in the line (vertical or horizontal),
- I subtract the number of pixels of the sprite
- Now I divide the number by 2 and get a "value"
- I put half of the "value" in both sides
You can also make a layer to use as reference, like the "Reference Point" layer that is inside the "All Animations in Aseprite.aseprite"
Don't forget to turn the reference layer to invisible before export the sprites