WordPress version: 3.1
WordPress theme: Twenty Ten 1.2
WP e-Commerce version: 3.8

Problem:

The “Insert into Post” button is missing when trying to insert a product image into a WP e-Commerce Product Page

Solution:

File: root/wp-content/plugins/wp-e-commerce/wpsc-admin/includes/display-items-functions.php

Line 1102:

Change

td.savesend input.button {
display: none;
}

To

td.savesend input.button {
display: block;
}

WARNING: I have no idea what type of consequence this edit will have on your WordPress installation or if it will conflict with your theme.