With almost all templates you can go into Admin -> Current Theme -> Themes and Settings -> (Click Theme Name).
If you want to change the logo.
On this page under "Theme Options and Preferences" there should be a text box option for "Logo image URL". Put your logo into your smf directory and put the name of it into this text box to replace the logo with your logo.
If you want place a html banner.
On this page under "Theme Options and Preferences" there should be a text box option for "HTML code to be displayed at the top right". Put any extra html for the banner that you want in that textbox. Also if you don't want it to show the logo as well then select the 'Don't show a logo at the top' check box which is just above.