When I created Buy Both Now and Buy The Set for Zen Cart, I never took account of the possibility that templates would use the CSS buttons feature, which is available in Admin->Configuration->Layout Settings.
I have updated the help for these mods to show how they can be adopted to use CSS buttons too. It’s easy – for Buy Both Now, just modify
includes/functions/extra_functions/checkbbn_pi.php
and change
$bbn_string_end = '<input type="image" src="' . $button . '" alt="Buy both now" title="Buy both now" /></form>';
to
$bbn_string_end = " " . zen_image_submit($image, "Buy Both Now") . "</form>";
and similarly, for Buy the Set, just modify
includes/functions/extra_functions/checkbts_pi.php
and change
$bts_string_end = '<input type="image" src="' . $button . '" alt="Buy the set" title="Buy the set" /></form>';
to
$bts_string_end = " " . zen_image_submit($image, "Buy the set") . "</form>";