What does the hash table store? Each tile's value? If so, I can only assume the tiles are in some sort of array to begin with, so you would be able to
i = 0 j = 0 for each row for each column hash_table.Add( ((i * numCols) + j), tile_array[i][j]) j++ i++
And since your keys are 0-255 with no gaps, why do you need a hash table? A 1-d array would serve you just as well...
Edit: from reading your previous question, I take it you are trying to generate one of 47 unique ids based on a 3x3 pattern?