How to change The top picture thingy

I assume you’re using the default template:

First, create an image and name it personalheader.jpg. Put it into your wp-content/themes/default/images directory.

Second, open header.php in a text editor and remove the /* and */ above and belowthe line:

#headerimg { background: url('<?php bloginfo('stylesheet_directory'); ?>/images/personalheader.jpg') no-repeat top;}

Then your header should be inserted properly.