Change Topmenu button

Hi there,
i tried to edit my topmenu button from text using the background-image, but this is the problem, I want to make a different image for each button, for now I have 3 main button, the first button id is menu-item-20, 2nd is menu-item-19, and the 3rd is menu-item-18, from the style.css, i tried to make it directly using background-image command like this:
.menu-item-20 { background-image:url(“images/a.png”); }
and inside text properties I add command like this
.menu-item-20 a{ color:transparent; visibility: hidden;},
the result is, for the active/currently page, the image on the button isn’t shown, and for the other buttons, the button doesn’t work anymore, so then i tried to edit the background inside the text area, .menu-item-20 a {sameasbefore}, the button work, at the currently page the button shown the image i add, but the size of the image is just like the text, and even more, it still have the text that I want to remove
can anyone help me? thank you very much!
Andrea