The reason you need to upload your artwork files to creativehub, as opposed to directly on your website, is that we print and fulfill your order using this print file. If the file is not in your creativehub account, we cannot print the order.
For this reason you should upload files to creativehub that are high res, ideally 300PPI and sized to your largest required print size.
Once you apply the sales settings to your files, our system generates a unique SKU code for each product or variant created. Learn more about how SKU codes work.