Change background image of Page Menu bar (for noobs)

Hi all,
I've already looked through the this forum and find many post refering to the issue of changing the background image - but i'm a noob and do not understand it. Sorry!

I have made a menu image with the dimensions of 1023x47 px. Saved as .jpg -but to which path/folder should I upload it?

How do I make it work as a background image lying underneath the menu navigation (I simply just want the navigation to be on top of the image, so that you can see my image through)?

Please help....

my page is: www.handyhansen.dk