Your images aren't part of Zen Cart and so their permissions will be a combination of what you uploaded them with and how your host treats them by default.

I'm not familar with PoweWeb or the tools set that they give you to look at files in your hosted area. Most hosting control panels would have some sort of file manager which you could use to upload files, or the FTP program you used to upload them will often have this facility.

Another possibility might be that PowWeb have protection in place that is intended to prevent people linking to your images that is erroneously being tripped when your Zen Cart tries to use them.

Sounds like a conversation with PowWeb's 24/7 Support may be the quickest way to carry this forward. They've got live chat so it needed cost you anything.