The algorithm for the tracer requires that the image you want to trace cannot touch the edges of the image. Hence the black border around the outside. By using a paint fill, you can then fill the area between the black border and the slab with white. When the tracer is told to look for only white pixels (255,255,255), it creates a profile around the outside and another one around the inside. The inside profile is the one you want. Also, the import uses pixels as units. If you are using inches, this may be 2,000 by 2,000 inches as a result. You may need to zoom extents to see the complete results.
As @DaveR noted, regardless of how you trim the image, you will get a much more realistic result if you start with an image that is square on to the face. Having some sample edge images help add to the realism.