You can choose between two display modes for the image included in your In-App message. This can be done from the dashboard settings, in "Themes":
Cover : Batch will use the entire width of the screen to display the image. In portait mode, depending on the screen size of the user device, the bottom and the top of the image will be cropped.
We recommend a square file, with a width of 1200px. Make sure the important part is not in a zone that may be cropped (the red zone):
Contain : Batch won't crop the image vertically.
You will now have two empty spaces on the left and the right of your image.
if you choose the Cover mode, we recommend you use the same color for the background of your In-App message and the background of your image.