Well, here goes:
The hex number is derived from three seperate hex numbers, giving it the variable representation of 0xABC.
Now, the A is derived by splitting the pics1.png file into 16 sections, like so:
Then, we divide each section into 256 peices, by tile, like so:
The tiles of each plane start at zero. The B variable is defined as the Y plane, and the C variable being the X plane. So, 0x001 would give you the second side of the first row, the right blade of grass. Right? Wrong! You also delete beginning zeros, to give you 0x1 as the answer.
Here's another one:
0x1FF. What is this? Well, it would be the #1 section of Chart A, and the last tile in the last row in Chart B, giving you empty grass.
Did you understand all that, or is there a point I need to clarify?