Start with your cleanest source
Use the original artwork, not a screenshot of a listing or a preview with a checkerboard baked into it. A visible checkerboard is not proof of transparency. Keep a separate untouched master so the AI result can be compared and replaced.
Choose one representative object with fine outlines and flat color. Begin at 2x. Increasing the scale increases pixel count, but cannot guarantee the artist’s original linework or a specific print quality.