I have another watermark image plugin that needs to access the Image Preview Upload+ full sized image and replace both the original and thumbnail.

Because it now must select the thumbnail how can it open the original and after saving replace both?

Hello,

The inserted code contains a link to original image. In fact it is stored in the same directory where preview is stored but its name is without "_small" suffix before extension.
You can add watermark to both two image (preview and original) and save them.