Why JPGs need extra care
JPEG compression stores images in 8×8 pixel blocks and smears colour around sharp edges. A tracer sees those smudges as extra colours and extra shapes. SVGShift applies a light selective blur before tracing and merges near-identical colours, so a scanned logo comes out as a few clean shapes instead of hundreds of fragments.
Scans and photos of drawings
For pencil or ink on paper, use Black & white mode. Move the threshold slider until the paper disappears but thin lines stay solid. Photos taken at an angle trace better if you crop and straighten them first.