Images not uploading when making new product

Hello there!
I have a client with a zen-cart shop that we set up.
She originally sent us an email saying "anytime I go into the system to add pics the product disappears".

In order to try to diagnose this, I added a product with 0 stock so it wouldn't show up, choosing an image to go along with it. When I proceed to the Preview stage, the image shows up as broken. If I then use the Back button (in the page, not the browser), it doesn't think I've attached an image at all. If I were to try to upload one, go through Preview, ignore the fact it appears broken, and add the product, it has no image.

I have a feeling that whatever is causing this may be causing the problem the client ran into.

Her cart was v1.3.9b up until this morning, when I finished picking all the files I needed to upload in order to update her to v1.3.9h.

We do have Zen Lightbox installed, but it's not working, either. Just tries to open the white box, then never loads. If I right click the thumbnail and open in new tab, the larger image IS there.
I'm hoping that whatever this image problem is, it is all related and hopefully fixed with one fix!

You guys are great; I appreciate any help you can give me.

v1.3.9b to v1.3.9h involves adding changed files and removing ones that are no longer required

Insure that the admin files set is ALL current and proper

Is there any reason not to just re-upload ALL the files from the latest version into my admin folder?

Is there any reason not to just re-upload ALL the files from the latest version into my admin folder?
Only if you have added any module(s) that have altered these files

Turns out the problem was a totally custom module that had been installed that I didn't know about, and had overwritten some of its files. I got it fixed!