As a general rule, your images should be at or around 72dpi. This will keep page load and data sync times quick. The larger the file size for each item's image the longer the pages will take to load and the longer things will take to sync.
Avoid uploading any images that are 150 - 300dpi.
Square crops tend to work best for product item images.
Ideally your product should be mocked up or photographed on a simple background for an easy read. Solid color backgrounds that contrast the product will make the item stand out.
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