On a related note, CalcGS is really picky about this stuff. I know I have a lot of trouble importing tiles, as they're often mangled, bleed over into other tiles, or replace other tiles altogether. Here's what I've found that works best:
- make sure the tiles are aligned evenly, and the bitmap resolution is even
- make sure the tiles are arranged top to bottom
- if you're using grayscale, don't even bother with dithering
- make sure you specify the exact bitmap resolution when importing, otherwise the tiles will be mangled