Replacing a search or submit button with an imageYou need to change a line in your code that looks like this:
<input class="searchbutton" value="Search" type="submit">
<input name="submit" class="cat_button" src="/images/button.gif" type="image">
Where /images/button.gif is the path to your image.
You can also style your button wuth just pure CSS and there are a couple of great links below showcasing and explaining how you can do this.
http://speckyboy.com/2009/05/27/22-css-button-styling-tutorials-and-techniques/ - A mix of different CSS button styles.
http://webdesignerwall.com/tutorials/css3-gradient-buttons - Using CSS 3 Gradients to create amazing button and hover styles.
<button></button> Works the same as your input but gives you more options in how you present your button.