You can attach an image, a GIF or a video to your push campaign from the campaign editor if the OS supports that feature. Click "Advanced" below the message preview:
​

🚧 Important note: On iOS, you will need to integrate an extension to add support for rich push notifications (more information here).

Here is some information on the supported attachments: 

Batch lets you send large-format notifications with a large image attachment. We require a landscape image, PNG or JPG, with a minimum width and height of 300px.

Images can be displayed on:

Here is how these notifications look on iOS and Android:

☝️ These formats work exclusively on iOS.

To add an audio file to a push notification, go to Advanced Settings > Custom image, audio or video and just click on "Use an URL" :

Select "Audio" and fill in the URL where the audio file is hosted:

💡 Note that the file must be an mp3 file with a valid mime type, hosted on an HTTPS server. The OS will automatically download the mp3 file and drop the download if it takes more than 30 seconds.

You can add a video attachment using an mp4 file, with a valid mime type and hosted on an HTTPS server. The video will be downloaded automatically on your users' devices and iOS will drop the download if it takes more than 30 seconds.

You can also attach a GIF file to your push notification. The GIF file must have a valid mime type and be hosted on an HTTPS server.

Here is how a video attachment looks on iOS 10:

If the attachment is not displayed, here are some suggestions to find the issue:

Step 1:
​On iOS, make sure you have implemented the additional extension documented here.  
Also, you will need to check the deployment target of the RichNotifications extension, here. If your device's iOS version is below the target, you won't receive an image in your push notification.

Step 2:
In case you are still facing the same issue, try disabling the energy-saving mode on iOS or Android and retry using a 4G network.