You are correct that NL doesn't store your photos (those remain on your computer until you delete them), but it does save the listing template itself indefinitely. If you store your photos in the same folder/directory and do not delete or move them, then NL does also keep track of the location of the photos, ready to upload to ebid with the listing info.
Perhaps you are referring to the fact that listings with photos on ebid itself will eventually disappear unless you log in periodically and do not let them expire?