Android 4.1 and newer devices support a BigPicture that will show below your notification text when it is expanded. But to not see your image cropped you should follow some rules.
The big picture can be located in your drawable folders, your assets folder in your app, or it can be loaded remotely from a server with a URL.
According to Google I/O 2012 (Page 77)
Images should be ≤ 450dp wide, ~2:1 aspect Bitmaps outside these bounds will just be wasting RAM (and possibly exceeding Binder IPC limits)
But it’s common to crop the image if image is a 2:1 aspect ratio. To prevent crop main content should be in 43:24 ratio (~1.79) as some devices crop past this width.
Android has not a size limit however onesignal recommend minimum, balanced, and maximum sizes below:
- Minimum - 512x256
- Balanced - 1024x512
- Maximum - 2048x1024