When a customer visits a product page in your online store, the first available variant is selected by default, and the image for that variant is shown instead of the featured image if you have one uploaded.
This is generally expected as the first variant needs to be selected in order to display a price for the product. If the featured image were to load first, the product price would not show until after a variant is selected. This is expected behaviour, because products with variants do not have a featured price. The price that is displayed is determined by the variant that is selected.
Modifying the theme code to override this behaviour and display the featured product image until a variant selection is made involves some additional development to the theme.
While this functionality is not built-in to the theme, Shopify has a customization tutorial.
However, we recommend reaching out to one of our Shopify Experts we've partnered with if you're looking to make this sort of modification as we cannot assist from a support perspective.