I have seen lots of people saying they wish they could change the header logo to something that more matches the design of their profile, well i have worked how to do it.
It's not part of the basic profile designer (where you just pick colors for different sections and enter them into the inout boxes) you have to do this via the "CSS (Advanced users)" section
What you will be doing is replacing the image of the ccs class called .header_logo
paste this into the css box:
.header_logo {
background-image:none;
background-image:url(http://url-to-your-hosted-logo-or-picture);
background-color:transparent;
height:60px; /* you change this px number to the height of your logo or pic */
}
An example of what this does can be seen on my profile here: http://www.phreik.net/Badly_drew_boy
Note if you have an img that is really long, you will probly cause your profile to expand really big and look stupid, so you have to play around with it till you get it how you want! Any trouble doing it, message me and i'll try and help ya!
Hope this helps some of ya out! 
Tags: Profile Logo Design Css
You could always disable html posts in your guestbook?!