This plug in is great, I was playing around with it and it took some tweaking but eventually I got it to work. It could use a few features, like a back end to pull the saved images from instead of having to track back through folders on the server to grab them. Also, the 'Add to Cart' didn't work right away because the text box 'Image Details' was required and the Image Editor wasn't putting information into that box. I solved that by opening the .js file and changing the text box name it was originally looking for, to the correct one it should have been looking for. I'm still having a problem with the thumbnails not working in the cart, but all in all, not too bad!
Some great features to add would be the ability to add text to the uploaded pictures (meme style). Also, whenever you add your image to the cart, that image should be totally wiped when you come back to the upload/editor. As of right now your previous image is there, it's slightly confusing.
This thing, with a little more development, is going to be KILLER. Please reply if you have any questions!