I tested on the schools computers, both on vista and windows 10 with chrome and IE same thing happens. the window for uploading the image does not show on any of them.

I can reproduce a problem when disabling WYSIWYG temporarily, then clicking the non-WYSIWYG "Add image" toolbar button.

I'll look at that, but otherwise I can't reproduce. I'm 50:50 on whether or not this is the issue you found.

Ok so I've fixed the issue I found. If you want to test, upload this new themes/default/javascript/editing.js file:
https://raw.githubusercontent.com/ocproducts/composr/d2a4a133e2062c642639f22af5e114c8f8142540/themes/default/javascript/editing.js

If that's not the issue, recording a little Jing screencast would help.

I did try and reproduce with WYSIWYG on in Windows 10 on Chrome and IE and it seemed okay.

Yes! your fix solved my problem, many thanks