Square images work best! If you have a Twitter icon, those are great. Just download your Twitter icon and upload that to Visible!
To change your company logo you will want to start in the desired entity. From here you will need to click "Settings" on the left side. Once you are in the "Settings" section you will need to click the "Profile" tab on the top of the screen. From here you will be able to change your logo. An example is shown below: