To create a Large Image Notification log in using your credentials and then you'll be moved to the dashboard. 

Then click Notifications > New Notifications.

Enter the Notification title, message, URL and change the logo.

Then enable the Notification Large Image option and then click on Insert Image.

Here you can add image URL from web else from your desktop. 

Make sure the size of the image is not more than 360*240px.

After adding the image your notification would look like this:-

You can also make a Multi Action Notification using Large Image Notification.

Click on Multi-Action Notification and add the title,URL, and image that you want.

Your notification would look like this:-

You can then send the notification to the users rightaway or can schedule the notification and send it later.