The simple answer is security. If you are planning to deliver a digital copy of the same image (the same file) that you are displaying in NextGen gallery then ask yourself the following question?
"Why would anyone buy the digital image when they can simply copy it from the browser?"
The image you display in NextGen gallery can easily be copied which is why you want to display a lower resolution and watermarked version of the actual image in NextGen gallery and configure eStore to deliver the proper image (stored somewhere else on your server or on an external server) after the purchase.
This is what I do...
1) Resize the images I display using the NextGen Gallery to a smaller size.
2) Apply watermark to the image using the NextGen Gallery "Effect" option.
This prevents stealing of images from the preview you display in the NextGen Gallery.
I store the real high res and watermark free version on a separate location and use them as my deliverable. Once the purchase is complete the encrypted download link for the original image (1920×1200 resolution and no watermark) is emailed to the buyer.
Now, this means more work for you as you have to configure every product but it's more secure. Most of the time your digital images will have similar details and variation so you can just create one product then use the "Copy Product Details" feature to copy and create a new product in one click then change the name and the URL of the image to be delivered.