Hey @Mélanie,
To do something like this you would have to write custom CSS on your site. We cannot help with writing custom CSS, but I can recommend checking out a [free!] tool like Firebug if you use FireFox, or the Developer Tools for either Safari or Chrome.
Each tool listed here has an “Inspector” tool that lets you zoom right over the element that you would like to customize – so, for the Map View, you can find where you want to put the background image, and then use these tools to determine the CSS you’d need.
If you are not familiar with writing custom CSS, there are fortunately many free resources online where you can learn quickly. A great resource, for example, is http://css-tricks.com
Best of luck with your customizing!
George