In order to change Rocket.Chat logo for your own (logo of your company), please, navigate to Administration -> Assets. There you will be able to change logo, login background and icon: 

1. To change the logo (the one that is located under the chats list at the bottom left side) click on "Select file" opposite the first field (logo (svg, png, jpg)) and select the corresponding file from your library. After this click on "Apply and refresh all clients" to finish the process. 

2. To change the login background click on "Select file" opposite the second field (login background (svg, png, jpg)) and select the corresponding file from your library. After this click on "Apply and refresh all clients" to finish the process. 

3. To change the icon click on "Select file" opposite the appropriate filed out of 4 given (depending on the file type) and select the corresponding file from your library. After this click on "Apply and refresh all clients" to finish the process. 

Source of the logo: https://dribbble.com/shots/3792673-Open-Logotype