Okay I managed to change it to a CSS button, not sure if it's the best way to do it but it works for now.
Stevesh was right, it was hard coded in the COWOA file.
I went into tpl_no_account_default.php, at around line 31 to line 34 (it's a div called #checkoutButtons) I replaced with this code:
Code:
<div id="checkoutButtons">
<div id="checkoutButton" class="buttonRow forward"><?php echo zen_image_submit(BUTTON_CONTINUE_ALT); ?></div>
<div class="buttonRow back"></div>
</div>
That basically stopped it using an image, so now I had my button but no text on it. To add text I went into html_output.php, at line 339 there is a value parameter that has no value, I just changed it to:
Code:
$css_button = '<input class="' . $mouse_out_class . '" ' . $css_button_js . ' type="submit" value="Continue"' .$text . '"' . $tooltip . $parameters . ' />';
This may change the value of other submit type buttons but I am not aware of that yet. I will keep updated.
Hope this helps anyone who might run into the same problem.