Apple allows macOS users to set the loginwindow image by replacing the PNG at
Previously, any PNG image would work, but as of 10.12, Apple requires the PNG to have an Alpha channel. This is somewhat annoying, because you need to either have photoshop installed, or a CLI tool like imagemagick.
You can use the form below to convert an image to a PNG with an alpha channel.
Upload a PNG or JPEG image you would like to set as your macOS loginwindow image.
Note: The conversion will take a few seconds, the process is running with severe resource limits on a small Google Cloud Platform instance.