The payload.zip file which you upload custom scripts with is where you also upload your branding.
You can customize the appearance of the software by adding a banner (400 x 125 pixel PNG) and an icon (256 x 256 pixel PNG) to payload.zip.
To do so, create a file named icon.png which is exactly 256px x 256px and in PNG format. Then create a file named logo.png which is exactly 400px x 125px. Put them both in the root of your payload.zip file and upload the payload and rebuild the MSI.
If you don’t have your images in the right format or size, we have used https://resizeimage.net/ and have been really happy with the results.
Please note: YOU MUST PUT YOUR FILES IN THE ROOT of payload.zip and not in a subfolder. This is very easy to do inadvertently.