Hi there! That sounds like a wonderful idea! Anyway, I didn't put up an easy option in the theme so that the logo can be changed, but here's how you do it:

First, you must upload the header you want in a photo-hosting site (like photobucket, etc.). It would be best if you keep the width to 500px so the theme would still work okay. Get the URL code of the image, then:

In your dashboard, click Customize (or "Customize appearance," since Tumblr re-designed itself), then on the gray tabs on top of the screen, select "Theme." Then on the bottom, click "Use Custom HTML."

Here's the tricky part since the code could look intimidating. Find the code that says:

<div id="header">
<center><img src="http://static.tumblr.com/ls5fqgx/tkzll0b2p/head-tree.jpg"></center>;
</div>

Remove the part that says "http://static..."; up until the quotation marks. Please leave the quotation marks! So it'd look like this:

<div id="header">
<center><img src=""></center>
</div>

Now, place your new header's URL code in the same area, inside the quotation marks. Then hit save! :)

I'm really sorry if I explain it badly, but that's the simplest I could come up with. :P If you've got any questions, just ask! :)