0- 7 : Magic number "ITERFILE"
 8- 9 : Image width
10-11 : Image height
12-13 : Number of bytes per iteration value (NBYTES)

IF NBYTES=2:
14-15 : Maximum iterations
16-   : Data (2 bytes per iteration value)

IF NBYTES=4:
14-17 : Maximum iterations
18-   : Data (4 bytes per iteration value)