Upload a photo, cut out the subject, and set it on yellow. Pick butter, sunflower or mustard, then download.

Yellow and blue are opposites on the color wheel, so navy sneakers, denim, blue glass and dark bottles get the sharpest separation against a yellow field.
Butter yellow (around #fde68a) is soft and modern. Sunflower (around #facc15) is bright and friendly. Mustard (around #ca8a04) is earthy and suits leather, denim and vintage items.
White and cream items lose their edges on a bright yellow. Choose butter yellow for light products, or add a soft shadow so the outline stays clear.
Pick any shade in the Change Background Color tool, or start from a color below. Marketplace main images usually need pure white, so use the white background tool for those.
Upload the photo, wait for the cutout, then choose a yellow swatch or open the color picker. Download the finished photo as a JPG.