This means that you are leaving the <a> to have no content. It's hard to hover above somthing that is so small it isn't visable.
You need to position the entire <a> not just the image.
However, if you are using images as the links you have another problem too. You are placing an image above the background, thus you don't see the changes in the background becuse your image is covering it up.
The possible solutuions I can think of right now are
* Use text instead of images.
* Use an image with transparent sections through which the background shines through.
I've been looking on the internet all day, and it's amazing how little information there is regarding this topic!