Its not that easy to change form element's style.. each browser has it's own way to render and create those elements. Some browsers will accept having a background image inside your select, others won't.
A simple technique is to add a default color, so the browser will render the color if it cant render the image.
background: #c3c3c3 url(http://www.3dtuning.ru/img/design/gallery-back.png) repeat-x 0 0;
so this way, firefox users will see a background image (something that always looks ugly, in my opinion) and the rest of the browsers will display with a gray background.
Hope this helps