When I want to replace an image with another one but want to use the same filename, there's the old thumbnail displaying in the panel > gallery.
- delete old image in panel
- copy new image from another folder in panel
- rename new image to old name
-> thumbnail changes
Clicking on the image shows the correct file and on the front end everything is fine, too, but that's really confusing. Actually it took me a while to realise only the thumbnail is wrong, that was my most frustrating experience with X3 so far.