OK, first you need to add a rule that is:
Code:
#searchContent input{
border:none;
background: url(path to your image);
}
I don't know the path to your image so you need to insert it.
Then you will see that this effects both the inputs in search. So, you need to discriminate between them. You can do this with some clever css or you can add a id to the text input box which I think is the better way to go.
To do that you need to edit tpl_search.php in includes/templates/yourtemplate/sideboxes/. If you want help with that edit let me know. If you want to do it that way.
If you want to try the css way then try:
Code:
#searchContent input[type=text]{
border:none;
background: url(path to image);
}
I think that has some browser compatibility issues with, you guessed it, internet explorer. Possibly only IE6 and earlier but I am not sure.
Best route is to do the edit of the php file.