add this line in body array.
'large_icon'=>"your image path",
@ravi_sojitra.Thank u so much.I am very glad for discuss this section with you…
Happy to help.
You can accept this solution so that it can help others. 
I have another doubt.My mobile device top notification section received bell icon.why?
can you show me how did you include that line i gave you in your code?
Here is my code.
let body = {
app_id: “5d4fd62b-b9f8-4e41-9a89-7ea3b18af3de”,
contents: {“en”: “English Message”},
included_segments: [“All”],
large_icon:“https://firebasestorage.googleapis.com/v0/b/apartments-ea9e5.appspot.com/o/logo-1.png?alt=media&token=8839bbd9-f2dd-4b9c-84e0-ed65871a7530”,
};
Here is my output.
after i see the custom icon
i think you might need to setup icon for your app with ionic resources command.
Try this link.
I generate icons and splash screen.Still show bell icon.How can i fix this?
ya.i have images in project root folder
/platforms/android/res/mipmap-hdpi/
/platforms/android/res/mipmap-xhdpi/
/platforms/android/res/mipmap-xxhdpi/
/platforms/android/res/mipmap-xxxhdpi/
as i told, it needs to be drawable-hdpi or drawable-xhdpi for the onesignal plugin to create notification icon.
I think.i create a folder name drawable-hdpi ?
I have images are
drawable-land-hdpi and drawable-port-hdpi inside /platforms/android/res/
Not the folder but it should be image with name i gave you in image
I create the images drawable-hdpi or drawable-xhdpi .But still not fixed
it needs to be like this.
What sizes are these images?
Can you show me your config.xml file?