Hi pperpe,
Edit yop_poll_model.php and in return_poll_html function replace
$template = str_ireplace( '%POLL-VOTE-BUTTON%', '<button class="yop_poll_vote_button" id="yop_poll_vote-button-' . $poll_id . $unique_id . '" onclick="yop_poll_register_vote(\'' . $poll_id . '\', \'' . $location . '\', \'' . $unique_id . '\'); return false;">' . $poll_options['vote_button_label'] . '</button>', $template );
with
$template = str_ireplace( '%POLL-VOTE-BUTTON%', '<img src="the source where you load the image from" class="yop_poll_vote_button" id="yop_poll_vote-button-' . $poll_id . $unique_id . '" onclick="yop_poll_register_vote(\'' . $poll_id . '\', \'' . $location . '\', \'' . $unique_id . '\'); return false;">' . '</img>', $template );
Regards,
YOP Team