Custom header, logo upoad not working to replace text!

Hi! I have gone in and customized gridmarket to make a premium theme. I have only adjusted the CSS and not any of the PHP files. I am working on building a tutorial, got to the "upload your logo" for the header section, tried to upload a logo but it didn't work. It allowed me to do the upload and preview the image I uploaded but when I went to preview....just the text still. Any ideas? Thanks!!