To remove the URL in the header, try going to wp-content, themes, this theme you're working on, and in there you'll find header.php. You should find the URL, search for it with the browser find function. While in there, after </head> put your new picture. Look in index.php also and you'll se a few things in there you may want to change. Index.php is like the frame of the site. Change something in index.php and you change for all the pages. Same for header.php but the header sits on top of the index, in that after </head> you can put whatever you want for making the look of the top part of the site. But you may end up finding the URL in index.php instead of header.php.