Ha looks like you are going to be learning quickly!
For the website image, in general the site is created by using a template, and the template can have lots of different stuff within it. If the site is simple, then the image url will be right in the template itself. If you wanted to change that image, you would upload the new picture to assets/images I believe, and then manually change the link to the new image name in the relevant template.
For example:
<div class="col-lg-3 col-md-4 col-xs-12">
<a href=""><img src="assets/images/logo.png" class="logo" /></a>
</div>
There are other ways to do this, and more involved methods when you want to do things in a unique way, but this is the basic method for pics on a page.
So, go into the tree, and find the page you want to edit (we call it a 'resource' which is a more general term). Then make a note of the template which that page is assigned to, that's what you have to edit. Then, go to that template, find the image reference, and change it to your new image name, after uploading it of course. As long as the internal link is right, it will appear.
EDIT: Just to be clear, the image is likely in the template but could be in the resource directly. For example, imagine I make a site with three pages, one for me, one for my cat, and one for my dog, and I want a different picture for each page. The template can contain the common header and footer, maybe other common elements. But the resources themselves will contain the pictures of me and my pets, because those are the individual resource contents rather than the template.
HTH, and good luck!
[ed. note: nuan88 last edited this post 12 years ago.]