In XHTML, there are certain ways that tags must be formatted in order to "work". This may or may not be your problem, but here is how to fix some of them.
In XHMTL, all non-closing tags must be "self-closing". This means that they have to have a space and slash at the end. Here is the right "end" of your image tag listed above:
<br> must be
<br /> too. All meta tags must have the same ending, too, as they don't have a closing tag.
I'm looking at your page now, and by "header image" I assume you mean the large photo that says "Jo's Cafe"? I see it in Firefox.
Now, back to the validation errors.
There are a lot of screwy things going on with your site's code. You have single quotes in place of double quotes on all your links (which will work but bork on some browsers), and there is odd mouseover scripts on some of the links like the read more.
<a href='http://joscafe.com/2005/06/11/fun-stuff/#more-296'><a href='http://joscafe.com/2005/06/11/fun-stuff/' name='ext296' onclick="showHide(296,'http://joscafe.com/2005/06/11/fun-stuff/',this,'entry');return false;">Read moreÂ»</a>
You have two link refernces but only one closing tag, which means that the other link is running around without being closed - and what is all that onclick stuff there for? You don't need it.
A website is like stacking dominos and if you mess up with one thing, it can filter all the way through and mess up other things.
Be patient and go through and carefully check everything, or honestly, try another well tested theme, unless you did this one yourself - then fix it.
It's a great site you have going, but these things will only get worse instead of better and they don't "fix themselves", unfortunately.
PS: I love Star Trek and my hubby is my best friend - ain't it an amazing world!