How do i move a picture away from the boundary line?

[Galvantua_YT]

So im trying to make something and the image i am drawing is too close to the boundary line to draw. i have extended the boundary lines but i need the image centered inside the boundary. i hit the center button wen i went to change the boundary size, an nothing happened. i really want to finish my project, its my first one on gimp, andd its turning ot great so far. any help would be appreciated.