[RM2K3] PANORAMA IMPORTING

I recently completed a map remake of FFV's merged world (link) and now I want to make water as parallax/panorama which would just slowly move to the right but honestly I can't get it to work.

I tried to save as both PNG and BMP using indexed colours with permitted maximum 256 colours (I use GIMP of that's any help), and I even made the water tile into 320×240px and 640×480px images (with the same qualities as mentioned above) but that doesn't help either.

When I'm importing it's either "Unsupported PNG Image" (for PNGs despite being indexed colour) or "Invalid colour depth." (for BMPs).

Also, this is the tile in question:

Thank you. It worked.