The vehicles database uses the idea of a gallery of images instead of just a limit of 2 images like in N Scale. Every gallery of images has a unique identifier. Two different items can both 'share' the same gallery. If you edit the gallery, you affect both of those items.
When you copy an item, the gallery id is copied as well. Don't modify the gallery, unless you are sure to use the same gallery for both items (the original and the copy).
As this is unlikely that this is what you want to do, you will probably want a new gallery for the new item. Here is how to do:
To create a new gallery, you just need to edit the item and blank out the "ImageGallery" field, then save the changes (update button). This forces TroveStar to create a new gallery and set the item to use that new gallery. Just edit again the item and you will see a new gallery id, as well as an empty gallery. You will then need to add the image to that gallery. To do so, just click on the Gallery Editor button at the top of the editor screen.