Go to Settings > Account > Look & Feel on the top righthand corner of the asset bank. (See screenshot)
If you would like to change the existing background image (the set background image for your account login page), just hover your mouse over the existing one and you will see a plus sign. Click on it and choose the new background image. Note that the image should be at least 1500 pixels in order to assure high quality.
This description is written based on a standard Bynder setup. The environment you use might have different settings, additional modules or custom changes. Thus, your particular situation may vary.