Hi,
After you create the characteristics size and color, along with their values, in the menu Products>characteristics, and you add the two characteristics to the product, you can go in the "variants" tab of the product to add the combinations that you want for that products and then you can edit each variant, and set its price and its image.
That way, on the product page, the system will automatically switch the price and the image based on the color/size selected:
demo.hikashop.com/index.php/en/hikashop/...with-characteristics