One of the most recognizable things about your app is its App Icon. You associate the phone app with a green background and a white telephone symbol. This case is the same for all of the other apps on the App Store. As you might expect, apps on the App Store are required to have an app icon in order to be submitted.
There are many paid apps that allow you to do this, but this article will show you how to do it for free using the built in Mac app, Preview.
Firstly, Take a screenshot using command+shift+3. Now go to your desktop, right click on the screenshot, select open with, and click preview. You should see something like the below.
Now hold Shift and drag to make a square, then click crop. Click the markup button (looks like a pen with a circle around it). Refer to the images below for clarification.
Now type command+a and press the delete button. You should see something like the image below.
Use the square, shapes, and other features to create an icon for your app.
Remember to press command+s when complete.
Now head over to https://appicon.co to convert the image into all the sizes needed. Select your app icon image and click generate.
Open the generated file in finder and expand it. In Xcode, remove the existing app icon file and drag and drop it in Xcode.
You may need to reselect the app icon file in project navigator.
Finally, run your app and see your new app icon in action!