how to swap text nav for images

I want to alter the theme I’m using by adding images as backgrounds to the text nav links but I can’t workout where I add the ‘background image’ tag in the style sheet, I have tried just changing the background colour of the area where the navigation sits but it seems to sit about 5 px’s over the right side of the main layout?? basically I just want to brighten up the buttons, I’ve tried adding images in the code of the head.php file and the position and alt text appear but not the images?
I’m getting very confused with this one.