To validate your web pages, you must include a header at the top of the page. Don't worry about the details of what this means, but just include it as the very first 4 lines, and change the simple <html> line as shown.
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
To validate the HTML in your document, submit it to http://validator.w3.org/#validate_by_upload
To validate the CSS in your document, submit it to http://jigsaw.w3.org/css-validator/#validate_by_upload
You can search for and download images from the Google Image Search page. Alternatively, you could just use the Birkbeck logo from the top of this page. If you don't know how to download and save an image file, ask a demonstrator.
Using CSS properly with a bit of thought about your initial HTML, you can totally change the look of you page without making any changes to your HTML. Take a look at http://www.csszengarden.com/. This page shows alternative views of the same HTML document using different CSS. Somewhere on the page, you will find links that let you switch the CSS.