Is this the 2.8 version of the tool?

It could just be another case of images going missing in the cache, not much one can do about it short of becoming a Ren'Py dev.

Could also be due to how long the filename is, maybe? Works for me on W10 and version 3 of the tool, but your setup may differ.

@Jman There's a version 3? of the tool?

Indeed.