Ok I fixed the issue, it was pretty simple too:
After updating to K2store 3 or updating Joomla 2.8 i found my add to cart button had been switched to an image.
I had to go an edit addtocart.php found in :/templates/gk_mystore/html/com_k2store/mycart
and change one word:
switch ($this->params->get('cartbutton', 'image'))
to ($this->params->get('cartbutton', 'button'))
Final code
switch ($this->params->get('cartbutton', 'button'))
{
case "button":
?>
<input onclick="<?php echo $onclick; ?>" value="<?php echo JText::_('Add to Cart'); ?>" type="button" class="addcart button" />
<?php
break;
case "image":
default:
$image = JURI::root(true).'/components/com_k2store/images/add-to-cart.gif';
?>
<img class='addcart' src='<?php echo $image; ?>' alt='<?php echo JText::_('Add to Cart'); ?>' onclick="<?php echo $onclick; ?>" />
<?php
break;