Hi,
Steps to display the search button as an image:
1. Log in to your websites Joomla Administration Panel
2. Go to Extensions- Module Manager
3. Click on the Search Module (which you have activated)
4. On the right hand side in the Module Parameters panel set the parameters as shown below -
Search button - Yes
Search button as image - Yes
You can set the Position of the Button
5. After making the above change, click on save.
6. After that take your image which you want to display as a search button and place the Search image in the /images/M_images folder and name the images as searchButton.gif
7. Now you can refresh the front end of your website, you will see the search button images displayed