Images:
The maximum dimensions allowed for an image is 2048 x 1536 pixels, with a minimum dimension of 1024 x 768 pixels. There isn't a resolution limit (just a size limit of 25MB), but we do recommend using the 2048 x 1536 size -- bigger is better.
Images under 200k can be uploaded, but a message will appear, warning the small size will result in poor quality images.

The best aspect ratio for a Showcase Ads image is 2:1 (width:height). This will fill up the image space in the ad most effectively.

Support: Zillow Help Center (https://zillow.zendesk.com/hc/en-us/requests/new)