Vertical images are ideal for short videos or animations, such as TikToks, Instagram Stories, and Facebook Stories, but for still images, I find that a square is best because then you can also read the caption under it! (And Instagram posts are square to start with.) This was just one example of a type of promo, there are definitely ways to adapt to various platforms with different image sizes, such as for banners, profile photos, Stories, or to optimize to put into a Medium story.