If a picture is supposed to 'paint a thousands words', then a product image on your ecommerce site speaks volumes to your potential buyers (did I mix my metaphors there?).
There are a number of factors to consider with images, including their: size, quality, background, etc. Each can help to contribute to a sale and several of them can further help a customer make the important decision whether to buy or not.
Obviously the most important thing is to show the right image for the actual product, unlike this photo on Amazon I found..... Supposedly for a gas barbecue, but actually showing a white striped polo shirt.