That would indicate there's something wrong with the palette or size of the file. For size, you just have to double-check the pixel size and make sure it's the standard wideness and that the height falls within multiplications of 32. For Palette you should look up TilesetPal and make sure that a proper palette is present and everything is indexed. For further palette work you might want to lookup PalSuite.

And btw, this guide by Disguise still is an excellent read even though it's slightly outdated now. It focuses entirely on Paint Shop Pro 8 for palette application and color reduction, while TilesetPal sure is the better choice above anything nowadays.