You need to add the image to your header file and then reference it in the stylesheet... You're using a theme from the looks of it so make sure you edit the right ones. Make sure you back up any file you edit first!
In the header (ie catalog/view/theme/YOURTHEME/template/common/header.tpl), before <div id="welcome"> add a new line:
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<div id="yourpic"><img src="/catalog/view/theme/YOURTHEME/image/yourpicname.png" title="Your pic" alt="Your pic" /></div>
In the stylesheet (ie catalog/view/theme/YOURTHEME/stylesheet/stylesheet.css) at the end add:
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#yourpic {position:absolute; top:20px; left:370px; }
Upload the image to the directory you're referencing. Make sure the id matches the stylesheet one, and replace the filenames, title and alt with your ones, and just fiddle with the position (ie add more or less to the top or left in the css) until it's right.
For the size just do a screen grab and open in any design software, you can work out the size by creating a shape in the area you want there...
Hope that helps.
cheers,
Sam