WordPress.com does not offer the option to change the HTML head tag directly. You can target a specific post or page with CSS and the Custom Design upgrade though. To do it, you should look at the body tag in the page source and find out what specific CSS classes apply to the page you want to target.
For example, you can use "postid-1819" for the http://icraved.wordpress.com/2012/11/05/grit-strength/ post. Here is an example that turns the title red for just that post:
.postid-1819 #content .post h2 {
color: red;
}
I see that you've already figured this out and you used this CSS to change out the header image on that page:
body.postid-1819 #headerimage {
background: url('http://i1247.photobucket.com/albums/gg636/xoxbekxox/GritHeader2.jpg') no-repeat;
}
Nice work!