Sorry for late reply, work work work
If you not make use of the Extra images feature, the most easy way would be to use that. The static button type in admin settings has to be enabled for this to work
So, you upload a voting image into public_html/images/extra/
Once done, you already see, it auto generates a voting code, but it will not work for your needs, so lets modify the "extra" template
Open up skins/your_skin/link_code_type_extra.html
Now, replace everything in that file with this
HTML:
{$lng->link_code_static_button} <a href="{$list_url}/index.php?a=in&u={$button_username}"><img src="{$list_url}/images/extra/{$button_name}" alt="{$text_link_button_alt}" /></a>
<br />
<textarea readonly="readonly" name="code" rows="6" style="width: 90%;" onclick="this.focus(); this.select();"><a href="{$list_url}/index.php?a=in&u={$button_username}"><img src="{$list_url}/images/extra/{$button_name}" alt="{$text_link_button_alt}" /></a></textarea>
<hr />
If you use the extra images already, we would need to create a plugin for that