You could place an image in the background of whatever container has the search box...
<div style="background-image:yourimage.jpg">your search box code</div>
I used a quick inline style, but you could put it in your external css or <head> however you have it set.
Or are you talking about something else?
Could you post a link to what you have and give a little more info about what you are trying to do?
A search box is not an image. It's an HTML form input field with a submit button. Use CSS to style your search form.
Thanks for the Search Forms-link Nancy-just bookmarked.