The four last tiles are garbage for the game works with only 256 tile, the rest is just data which was there in the memory right then. Naturally, a 256x256 resolution image could be made and then there would not be the plus four garbage tiles, but then the "metatiles" would have fallen apart; the graphician - probably - worked with 320x208 images, thus they are in place like this.

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SHA1 checksum: b5fe7de11bd8e6d1b485a627e92b483952ed8dbf