There are two main sections where you can update your brand logo and/or product images.
Updating your logo/image on your listing
Go to the brand management 'Catalog' section (found on the left sidebar)
Click the listing where you would like to update the picture/logos
Go to the 'pictures' section of the details and upload the image(s) that you would like to use.
Click 'Set entry pictures' to save your update.
Updating your logo for your brand
Go to the brand management 'Brand Details' section (found on the left sidebar)
Navigate to the 'Logo' section and/or 'Brand Manager avatar' sections
Upload the image that you would like to use for both sections
Click 'Set brand logo' and/or 'Set brand manager avatar'