Problem changing Gridmarket Header logo for multisites.

Hi, I have a problem changing the default header logo on my website. I was able to easily upload and change it on my site, but I want the same logo to appear on the multi-sites as a default. For example, on the “Gridmarket” text is still showing. Where in the coding would I have to go to change this so when a user signs up and gets their own sub-domain to the site my custom header automatically shows up and not the “Gridmarket” text?? PLEASE HELP!!