Product image field best practice
If the image doesn't really vary by variation, I would put it on the product. This will save you disk space / database space in the long run. Alternately, you may be able to use the Media module to upload the image once and reference it from all of the variations. This would let you use a single product type to handle product pages whether their variations required different images or not.

At the end of the day, there is no single best option here.