The only way I can imagine doing it would be to see to it that the sprite does not fill the bounding box. The imported imaged would need to be small enough in relation to the bounding box. A lot of graphics programs auto crop on export. Sometimes I cheat by placing four small marks or dots in the corners of the image to set a crop size. I think it would be awesome if the image editor allowed you to scale the image and not the bounding box. It would also be really bitchen if the image editor could do onion skinning.