This feature available on All Plans
In ShortStack, our templates are made to be customized - you can remove or change almost anything you'd like while making your campaign. The templates are there as a starting point; design-wise, you're free to truly make the campaign your own.
That means that there really aren't any required image dimensions or sizes; you can design your campaigns to any specs you'd like. On the other hand, if you would like some guidance in image sizes, we have you covered there as well.