Start with skin1/modules/Fast_Lane_Checkout/home_main.tpl.
"checkout_0_enter.tpl", "checkout_1_profile.tpl", etc. are each of the steps in the checkout process.
The part after the "else" statement is the default "Big Buttons".
You may also check skin1/customer/main/cart.tpl but it would probably look better displayed next to the "Big Buttons".
Here is home_main.tpl with the added image (your filename may vary)
Code:
{if $checkout_step eq 0}
{include file="modules/Fast_Lane_Checkout/checkout_0_enter.tpl"}
{elseif $checkout_step eq 1}
{include file="modules/Fast_Lane_Checkout/checkout_1_profile.tpl"}
{elseif $checkout_step eq 2}
{include file="modules/Fast_Lane_Checkout/checkout_2_method.tpl"}
{elseif $checkout_step eq 3}
{include file="modules/Fast_Lane_Checkout/checkout_3_place.tpl"}
{else}
<div align="right">
<table cellpadding="0" cellspacing="0">
<tr>
<td>{include file="modules/Fast_Lane_Checkout/big_button.tpl" button_title=$lng.lbl_continue_shopping style="button" href="`$back_url`"}</td>
<td><img src="{$ImagesDir}/spacer.gif" width="10" height="1" alt="" /></td>
<td align="right">{include file="modules/Fast_Lane_Checkout/big_button.tpl" button_title=$lng.lbl_checkout style="button" href="cart.php?mode=checkout" color="red" arrow="Y"}</td>
<td><img src="{$ImagesDir}/spacer.gif" width="10" height="1" alt="" /></td>
<td align="right"><IMG src="{$ImagesDir}/quickssl.jpg" width="102" height="47" border="0" alt="Secured by GeoTrust" /></td>
</tr>
</table>
</div>
{include file="customer/main/cart.tpl"}
{/if}