In order to add a logo to the vendor registration you need, when logged in as an administrator, to click the Vendors button in the menu.
Click on the letter of the alphabet that corresponds to the first letter of the Vendor’s company name.
Click the Edit button and the Other Details box will be displayed:

If the logo hasn't been uploaded click Upload New File. If the logo has been uploaded at File Location, select the folder that the Vendor's logo is located in and at File Name, select  Vendor’s logo from the drop down list.