Actually... for sprites being added, I have an idea that would probably be a bit more precise... yet graphically easy.
Where it displays the image in a scroll box, you specify the width and height of the sprite you're adding, and you drag the 'cropping' box around the image, and click to add it. That way you don't have to worry about getting the exact coordinates of the box right. (As well as having a grid in the image as well, but I didn't feel like going through the drawing of that)
Also, I wouldn't mind, if * was in place on sprite number, having it added automatically at the end of the sprite list.