Since you have a store on the Platinum plan that allows for code editing, you have quite a few options here. Most of the default Big Cartel themes have jQuery included so finding a plugin that uses jQuery would probably be your best bet.
Here's one example of a plugin: http://www.elevateweb.co.uk/image-zoom
With this example, you'd just need to upload their jquery.elevatezoom.js
file to a separate server, and include it in your store's Layout file.
You can then edit the code on your individual Product page (using the Luna theme code as an example):
<img src="{{ image | product_image_url: "fullsize" }}" class="fade_in" alt="Image of {{ product.name | escape }}">
Can be changed to:
<img id="product_zoom" src="{{ image | product_image_url: "fullsize" }}" data-zoom-image="{{ image | product_image_url: "fullsize" }}" class="fade_in" alt="Image of {{ product.name | escape }}">
Then just use the following code to activate:
$("#product_zoom").elevateZoom();
Hope that helps!
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