I do have extensions visible. I already tried swapping a background from .png to .jpg just by changing the name, and it couldn't find the image. RPGMaker MV specifically said, "Can't find bg lionshead5.png". Upon changing it back from bg lionshead5.jpg to bglionshead5.png and pressing try, it immediately found it again.So you just need to change the file extension (.jpg to .png). Batch renaming programs support this.
Do you have the option to show extensions in your operating system enabled?
In Windows it is Explorer Options - View - Hide extensions for registered file types - Disable.
Tried replacing the last three scenes, everything works.
Swapped in the negative for clarity.
Maybe the editor will also de facto support jpg images if you just change their extension in the name to png?
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While it was called bg lionshead5.jpg, it wouldn't appear in the editor's picture viewer, so I wouldn't be able to place it.
I'm not entirely sure what's going on there.