The fields you need to replace an image don't appear in a new window. They are inserted at the top of the page, which is easy to miss if you're using the 'Replace' button at the bottom of the page.
I had the same problem; I was expecting a pop-up window because that's how images are managed within a listing. Oddly, support couldn't figure out what the problem was, even after I told them I had pop-up blockers turned off. They told me to reinstall my browser [!] and get back to them. I managed to solve the issue without their help.