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 account logo (set account logo) or the account login logo (set account login logo), just hover your mouse over the existing one and you will see a plus sign. Click on it and choose the new background image.
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.