[Victor D]

If you need more than one custom button you can do it like this

{elseif $style eq 'button1' && ($config.Adaptives.platform ne 'MacPPC' || $config.Adaptives.browser ne 'NN')}
 <table cellspacing="0" cellpadding="0" onclick="{$js_link}" class="{$custom_class}"{if $title ne ''} title="{$title|escape}"{/if}>
 {strip}
 <tr>
     <td>
         <{$img_type} src="{$ImagesDir}/spacer.gif" class="{$custom_class}-l" alt="{$title|escape}" />
     </td>
     <td class="{$custom_class}"{$reading_direction_tag}>
         <font class="{$custom_class}">{$button_title}</font>
     </td>
     <td>
         <img src="{$ImagesDir}/spacer.gif" class="{$custom_class}-r" alt="{$title|escape}" />
     </td>
 </tr>
 {/strip}
 </table>

Now you should apply custom styles:

/*FirstButton*/
TABLE.FirstButton{
/*common first button styles*/
}
img.FirstButton-l{
/*left part of the first button styles*/
}
img.FirstButton-r{
/*right part of the first button styles*/
}
TD.FirstButton{
/*center part of the first button styles*/
}

/*SecondButton*/
TABLE.SecondButton{
/*common second button styles*/
}
img.SecondButton-l{
/*left part of the second button styles*/
}
img.SecondButton-r{
 /*right part of the second button styles*/
 }

TD.SecondButton{
  /*center part of the second button styles*/
 }

....
/*TheVeryLastButton*/
TABLE.TheVeryLastButton{
....

Now you can include button.tpl as
{include file='button.tpl'.... style='button1' custom_class="FirstButton"}
{include file='button.tpl'.... style='button1' custom_class="SecondButton"}
....
{...}

is this the only .tpl that has to be changed when applying a new button?

If you include button.tpl directly this tpl is the only one.

One more thing, where is the reference to the midd img?

in CSS TD.button1 should be like this

TD.button1{
background: #fff url(images/btn-c.gif) left top repeat-x;
}