Go to the “Checkout for Guests”, “Checkout for New Customers”, “Billing Options” templates using the Source Editor, find

<ss:button type='submit' value="Add To Shopping Cart"/>
and replace it with
<ss:button type="submit" value="Continue Checkout" source="$templateSet.images['xxxxx.jpg']"/>

For the Ship to and Ship to Gift templates the new button code needs to be:

<ss:button type="submit" value="$address.buttonText" source="$templateSet.images['xxxxx.jpg']"/>