When you edit a module and upload a new photo, will that replace the existing panel photo, or add an additional one? There are a few I think I could upload, but I didn't want to replace anything that's currently out there.
When you upload a new photo it will replace the old one, but the old one still is not lost.
For every upload a new version is added.
Other people who have set their module image to HEAD version (default) will see your newly uploaded image.
HEAD means: always show the newest image.
If they switch to an older version it will stick there, newly uploaded images will not affect their racks.
On a related note, why are there so many alternate panel photos that are exactly the same image?
When you click on those arrow buttons technically you browse through the versions. But a new version is also added if someone just edits a spec like depth and does not change the image at all. The Panel Selector function was added to ModularGrid lately and is more a hack to make different images possible. It's not a well designed feature but a compromise.