Drop down triggering image_button callback
Found the problem with the code. Image_button won't have any #value. It will only have return_value which in this case can be same as what I was putting in #value.

    $form['fieldset1']['image_button'] = array(
    '#type' =&gt; 'image_button',
    '#name' =&gt; 'image_button_name',
    '#return_value' =&gt; t( 'Image Button Value' ),
    '#src' =&gt; drupal_get_path( 'module', 'my_module' ) . '/assets/img/image.png',
    '#submit' =&gt; array( 'button_callback_function' )
);