How can I change mockups on Shopify's product page using Fuel app?

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I am using Fuel as fulfillment partner for my POD business. When I create a new product on Fuels app the same gets automatically uploaded to my Shopify store. However, if I then want to change the mockups to my own ones by uploading media to Shopify Product Page, then the uploaded one does not show up on clicking withen the drop down boxes of colour variants available with that style. Is there a solution for this?