Im my site, the Fusion Core based theme, Acquia Prosper, adds a border with background and -moz-border-radius to every form items, like buttons and images.
this of course clashes with Custom Search image, as it is an input field.
so i faced the problem of having to remove those borders, background and border-radius from it.
i though no biggie... until i came to the conclusion that Custom Search module does not sets a class attribute for the image, although it does for the search box and search form.
so i wrote a patch for 6.x-1.4, 6.x-1.x, 7.x-1.4 and 7.x-1.x.
it is quite simple - it only adds a single line in all 4 versions for .module file:
$form['submit']['#attributes'] = array('class' => 'custom-search-button');
$form['actions']['submit']['#attributes'] = array('class' => 'custom-search-button');
basically it adds a CLASS attribute to the image input item. nothing else.
the class (custom-search-button) can be loaded in the theme .css files and is entirely optional.
i've tested 6.x-1.4 and it works fine.
the other versions i did not test, but they should work well as well.
patches for all versions supplied bellow.
i've also included the .module file with the patch applied for all versions.