You didn't get it...
There are 256 different tiles you can use, numbered 0-255. There are three big rasters (back, mid and front) containing 256x256 bytes. Each byte is the number of the tile that should be used there. Then there are the actual tiles. They contain a texture (the image you see on them) and several properties such as friction and breakpoint. These are also stored in the level file (so you can make tiles look like anything). But usually only tiles 1-250 (not 0 or 251-255 which are special tiles) are stored, with nothing telling how many are stored. With church1, my level parser noticed it didn't reach the end of the file after it had parsed it, and after changing it to keep reading tiles until the end was reached it returned 5 additional valid tiles (so that level contains tiles 1-255). See the big quote above for a description. Darkness contains 1-250 as usual.