The image might be too large. When you run a mobile app message with an image, make sure that the image resolution is not more than 600x300 or more than 40 kb in size.
These settings ensure that the image is rendered on low-end and mid-range devices. A large-size image with 2:1 aspect ratio might only render on high-end devices. Mobile app image rendering also depends on the device network and if the network is strong enough to download the image.