You should be able to just view the source of the webpage and then find that part of the code. For example, this forum page that I'm typing on has a b2evolution logo as well. By viewing the source I see the following code:

<div id="rowheader">
	<h1><a href="http://b2evolution.net/" title="b2evolution: Home"><img src="http://b2evolution.net/img/b2evolution_logo.png" alt="b2evolution" width="472" height="102" border="0" /></a></h1>

So I know that the image is called b2evolution_logo.png and it's in the /img directory of this website.

I'm wondering though, which skin are you using that has the b2evolution logo in the header?

I am assuming it is the dafault skin, since I have made no changes. I am a blog newbie, and am still working my way around it.

Try changing the skin to "custom" if you can. You do this in the back office under the Blogs tab. See if that takes you to a better starting point before you start hacking and slashing. You also might want to visit the skins page (there's a link along the top of the page you're currently reading)....There you can hopefully find something more to your liking.