You can show any image in notification by adding parameter "image" while sending parameter.
Using FCM with image is assets folder:
{"to":"[add your token]","notification":{"title":"[add title]","body":"[add your message]","image":"www/images/test_image.png"},"priority":"high"}
Using FCM with image in drawable folder:
{"to":"[add your token]","notification":{"title":"[add title]","body":"[add your message]","image":"ic_icon"},"priority":"high"}
Using FCM with image from external link:
{"to":"[add your token]","notification":{"title":"[add title]","body":"[add your message]","image":"http://www.test.com/test_img.png"},"priority":"high"}
Using GCM with image in assets folder:
{"registration_ids": ["[please dont change]" ],"data": {"tickerText":"example test GCM","contentTitle":"content title GCM","message": "Enter your message","title":"GILAC","image":"www/images/test_img.png"}}
Using GCM with image in drawable folder:
{"registration_ids": ["[please dont change]" ],"data": {"tickerText":"example test GCM","contentTitle":"content title GCM","message": "Enter your message","title":"GILAC","image":"ic_icon"}}
Using GCM with image from external link:
{"registration_ids": ["[please dont change]" ],"data": {"tickerText":"example test GCM","contentTitle":"content title GCM","message": "Enter your message","title":"GILAC","image":"http://www.test.com/test_img.png"}}
Note: Notification icon/image must be png image.