If you are asking how to change the site logo image, the answer is below.  If you are talking about the whitespace off to the right of the banner, then you'll have to do some custom code I imagine.

Go to:

Site Administration -> Appearance -> Themes -> (Click the theme you are using)

Once here, you will see a section for the Logo.  You will upload your logo here and Save Changes.

*Depending on which version of Totara you are on, if it's a newer version you'll most likely be using the default 'Basis' theme.

'Site Administration -> Appearance -> Themes -> Basis'