Bitrix24 Standard or Professional plans subscribers can add own company's logo to the Bitrix24 account. This option can be found under Settings in the left-sided menu:
Use "upload logo" tab under Company Logo section. Please note that the maximum file dimensions should not exceed 222px by 55px.
Remember that you can always delete logo and upload a new one if necessary.
The uploaded file will be displayed as your company's logo in the left top corner for all Bitrix24 account users.