What i do is go into the theme that im using (gallery/theme/chosen theme) in my file manager, then go into template.html on click the html manager and either anywhere on the template (eg.top) or where it says '{description}' or '{name}' i change this to some text saying 'adsense'.

Then save it and go into code editor, change 'adsense' to the google code and save.

while your doing this you should have the gallery open and refresh it everytime you do save, and you might need to fiddle around with the template for abit until it looks right.