Never mind, those three WERE about size.
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Another idea would be to make it so that downloading the combined sets wouldn't be a huge problem.
{setname}_{terrainpieceID#}.{image format}
So, for, let's say the horizontal pillar in the Pillar set, or whatever you call it.
pillar_01.png
Why *.png? No reason. Just an example.
This would allow for custom tilesets.
A good idea would be to set up a tileset thing somewhere, too, for people to post their tilesets.
Such as, computer game tileset clones? Like, cavestory-eggcorridor_01.bmp?
Or: supermetroid-tourianruins_01.gif
sonic3andknuckles_sandopolispyramid.jpg
It would work great, and take less space in the long run!