Hi,
1). Connect to your admin panel: http://yoursite.com/wp-login.php
2).Press on Appearence button in your Dashboard
3). Press on Header button in your Dashboard
4). Press on Browse…
5). Find image in your computer which you want to make your header.
6). Select it, press Open button
7). Then press Upload
8). Press Skip Cropping
9). Press final button; SAVE in the bottom of page
10). Check what you have in your front page
