Search button with 'lens' icon on the exposed view's filter - Drupal 7
The code you posted is good. The only "problem" is that your lens image is "sprites" image - designed to be shown only partially, depending on the state of your button. There is no standard for this, so Drupal does not support this. You ave two options there:


Cut your image, leave only the part you need.
Use CSS to implement sprite style your image was created for.