Tile/Sprite files should be in the same format as generated by the Image2Tiles tool. This program only works with 8x8 tiles. If your tiles are larger, create 8x8 versions of them to use as a reference in this tool.

PROGMEM const byte GRADIUS_LEVEL1_TILES[] = {
  0b00000000, // 0
  0b00000000, // 0
  0b00000000, // 0
  0b00000000, // 0
...


PROGMEM const byte GRADIUS_LEVEL1_TILES_PROPS[] = {
  0b00000000, // 0
  0b00000000, // 1
  0b00000000, // 2
...
        

_PROPS will be appended to the name of the variable holding the tile data and that will be the name of the properties array generated. Copy that into the same source file as your tile data and you can use your saved version to populate the form here to adjust settings. Just copy, paste and save the output from this program back to your file.