Gallery2:How to Add Custom Banner or Remove Gallery Logo
From Gallery Codex

If you want to add your own banner at the top of every page, you just need to follow this simple process:

1. If you don’t have a /local directory in your theme’s template directory create a local directory at /themes//templates/.
2. Copy the theme.tpl file from /themes//templates/ to /themes//templates/local/.
3. Finally, edit the copy of theme.tpl you just made and remove or change these lines:

url href=”sitelogo.jpg” forceDirect=true}” alt=””/>

Then put a sitelogo.jpg file in each multisite directory.