How to customize the search module???

Hello

I am trying to change the background of the search field, the one in the start page, but i can't find how. I tested with change the color of the fields in the file SearchBlock.tpl and it worked, but when i try to put a background-image using css tags the background-image is not commin up.

The specific line is this one:

original:
<input type="text" id="searchCriteria" size="18"

this works
<input style="background-color: blue;" type="text" id="searchCriteria" size="18"

this not

<input style="background-image: url(path to image);" type="text" id="searchCriteria" size="18"

Someone knows?

Thanks