To achieve the best possible image quality and assure speedy loading times, always upload compressed images with twice the size of the recommended sizes to accommodate retina displays as well.
You can also find the ideal image sizes in the upload modals after setting your mockup style, or you can download our mockup canvases by following these steps:
Hover over a Mockup section.
Click the Mockup Style button in its top-left corner (palette icon).
Click Download the mockup canvases in .sketch and .png at the bottom of the modal.