We currently support the following image aspect ratios in image app download cards: 1:1, 1.91:1, 5:2. These images must be at least 800px wide.
image_display_height response attributes will allow you to determine the rendered aspect ratio. These attributes will take the following values:
Note that these values do not represent the image dimensions. Rather, they represent the display dimensions.
For consistency with the previously announced changes to website cards, we will be transitioning away from the 5:2 aspect ratio. As early as 2018-01-04, we will no longer allow image app download cards to be created using 5:2 images.
Twitter Ads API Team