In each type of campaign there is the option of adding icon and large image.
The icon is displayed on all devices and usually represents your website logo or a profile picture. Some browsers (e.g. Chrome on Windows and Android) let you display a large image inside the notification content.
You can upload each file from your computer or provide the url.
When you upload file from your computer, you don't worry about HTTPs. We provide all your images and icons over HTTPS, otherwise some browsers (e.g. Chrome) may not display them for security reasons.
When you provide url all images and icons should be served from your website using a CDN. This is not only general advice. The point is that when a notification is sent to many subscribers, there are thousands of browsers that download your image every second.
Suggested size is 192x192px. Suggested format is PNG.
Suggested size is at least 750px wide. Suggested format is PNG.