To answer your question, you should be able to fix the issue by using or DIRECTORY_SEPARATOR as a variable in place within the code in various places. I don’t imagine this script will work overly well on windows. I think I rememeber seeing other spots in the code which would require the change. Ideally to make it cross platform I believe it should be DIRECTORY_SEPARATOR.
Have you tried these steps to fix it?
However if you want to remove the Additional Image form element you can try deleting it in the admin under Configuration > Ad Form click Additional image and click the Delete button. I don’t know how well this will work though. It may present other issues or it may not even let you delete it.