Well, what I do is just add the black stripes from the 3 count and 2 count to the sides of the 1 count. Makes it look nicer, but it doesn't fill in the gaps. Aside from doing what the above poster said, you could open an image editing program and add transparency to the left of the 3 and to the right of the 2 to "push" them toward the 1 count. Just a theory, but I'm not sure whether or not osu! has some type of limit to the size of the image.