So you decided that you want to use an image instead of styling the inputs for the Drupal search block or form. Luckily, this isn't a difficult task at all. Using the hook_form_alter you can accomplish this. You will need to create a small module in order to make this happen. I called mine "modified_search". Here is what the .info file looks like:
name = Modified Search
description = Inject the search how i want it to look.
package = Other
version = VERSION
core = 6.x
And now for the contents of the .module file: