You'll want to create a "Child Theme" for your website:
After you do that, you'll edit your header.php file to insert that camera image. But let's get the Child Theme working first.
Go into your server's file system where you installed WordPress, and find a folder called "wp-content". In there, find the "themes" folder.
Create a new folder called "twentytwelvechild" and enter it. Now, we're going to create a file called style.css
Open up a text editor and paste in the following code:
Theme Name: Twenty Twelve Child
Description: Child theme for the Twenty Twelve theme
Now save that as "style.css" and upload it your "twentytwelvechild" folder. Congratulations! You have a Child Theme.
One more thing, go back to your "themes" folder, find the "twentytwelve" folder. In there, copy the "header.php" file and paste it into your "twentytwelvechild" folder.
Please reply after you do this, or if you have any problems.