If you want to use background-images (I prefer background-images as well for navigations) you could absolutely position a blank image over it by adding this CSS: position: relative; z-index: 100;
to all of the navigation elements with background images and then putting this in them:
<img src="pixel.gif" alt="Text to display when images are off" style="width: 100%; height: 100%; position:absolute; top: 0; left: 0; z-index: 50;" />
Then, when the images are off, the alt text of the blank image will show. This image will be under the element, but when images are off, you will be able to see the image's alt text. Also, this will work for partially transparent background images.
You can use this pixel.gif image.
Hope this helps.