Plugin that adds product variation image swapping function to WordPress e-Commerce plugin (WPEC). Requires 3.8+ of WPEC plugin.
Remember that variations are in combinations, so all selects must be selected once for the system to know which image to grab for you. For example you have a product called T-shirt and it has 3 colors and 3 sizes. The user would select large and then red for example. Then the image for that combination will be retrieved. From then on if they just select a different color, it will also work as it will look for the next color with the size large.
Check the checkbox in the sidebar widget that says "Disable Variation Image Swap". and click on Update/Save. See screenshot 1
It is possible the theme you're using is not compatible with the plugin if the theme has changed any class/id names on certain containers. To know for sure, switch to 2011 default WordPress theme and re-test.