Login Image Transparent Background

Are log-in images with transparent backgrounds supported?

I was trying to set my login image through the Ultimate Branding plugin and it has a transparent background. For some reason, the plugin just replaces the transparent area with the color black. Do you know why this is happening? If it keeps replacing the login image as black, how do I make the custom login page look nice with a custom background?

Please respond as soon as possible. All help is appreciated.