If you click the "Valid XHTML" link in the menu on your site, it takes you to an XHTML validator, ie. a page that checks if you've followed the rules of XHTML properly, or if you've cheated here and there (or simply forgot to end tags, or misspelled a tag, anything that's not stricly correct). This is a Good Thing© because when everything goes completely by the book, there's a much higher chance that all browsers will be able to show the page, and show it correctly.
One of the rules of XHTML (as opposed to normal HTML) is that all elements must be closed (ie. have a </element> to close it). This includes single-tag elements (ie. one that in HTML only consists of <element>, not <element>content</element>) as well, but to avoid too much cluttered code, an easy way to close these tags (for instance an <img src="something"> tag) is to add an extra space and a /slash/ before the final >, ie. <img src="something" />. When you put in the picture describing what mood you're in, you didn't do this, you used the older HTML method, which is invalid in XHTML. Also, in XHTML, an img tag needs to have an alt value (ie. <img src="something" alt="text that appears if the image doesn't load" />) as well, and you didn't give one of those either (same image).
If you correct these two things and click on the 'Valid XHTML' link in your menu again, it should take you to the same validator, but this time tell you that, "This page is valid XHTML 1.0 Transitional".
(Sorry if I sound condescending or anything like that, I don't mean to - I just assume that you're used to using HTML and don't know much about XHTML)