Change titel background image

Hey,

I use the theme “customizable” for free (for now), but i can´t change the titel background image (the image with the big city: http://www.screencast.com/t/FTeKjwH4MnjB. How can I change it from the city-image to a image of my own? The city image is just not working with my site! 🙁

I had downloaded and activate the plug-in: http://ww.wp.xz.cn/plugins/simple-custom-css

And if I can use my own image – what is then the optimal size for the image?

You can se my site here: http://new.solvaenget.dk (Its in danish).

Hope someone can help me oderwise I have to change the theme of my website :(.

/ Sif