Go to: My Account >> Update User Profile Screenshot
You can use the logo/image that you may have already uploaded or click on the Image Library link to upload a new Image.
Screenshot
What are the benifits of setting up the Company Logo?
You can use this Company Logo on your Surveys, Email Invitation and also on Certain Reports. If you have an Organization account then the Main Account can upload and set up a Company Logo and this can be used across all Sub-Accounts and Dashboard accounts.