Greetings, and welcome to the forums.
Regarding the Mint Fresh Youjoomla June '07 reference, that is the file /images/Logo.gif. It is 393 px wide by 105 px tall. Simply replace it with a file of the same size and name.
On the reference to the Joomla books ad logo, I'm afraid I don't see that on the demo. Can you provide me with a link to your site, or something else that can help me locate what you are referring to? Thanks!