This is not strictly a custom template question.
Basically, I can think of a few options. All involve putting your own image upload form on the thank you screen so that the shopper sees it once they have completed their ShopSite order. You can have a hidden form field in the form that contains the ShopSite order number.
1) you can do it via the shopping cart template, in the DEFINE THANKYOU section you can use the template tag [-- ThankYouOrderNumber --] to get the order number to populate the hidden form field.
2) you can use JavaScript to populate the hidden form field. If the store is ShopSite Manager or Pro you should be able to use the ss_ordernum variable in your JavaScript code, see:
http://www.shopsite.com/help/10.1/en-US ... ables.html
You could put your form an custom JavaScript on the thank you screen via a custom shopping cart template, or you could put it on one of the text areas in Commerce Setup -> Order System -> Thank You.
3) if the store is Pro, you could use the Order API feature to create your own custom cgi to write your own HTML 'thank you' page that includes your form. And of course the order number is one of the values provided to the custom cgi via the Order API, see:
http://www.shopsite.com/help/10.1/en-US ... r.api.html
-Loren