Hey, you need an image manipulation software or something like that.
Gimp 2 is the free way, I wrote a little tutorial to help you out :)
1) Import the png file to Gimp 2
2) Set the grid size to 16x16 with Image -> Grid setting
3) Show the grid with View --> Grid
4) Use the selection tool and save the selection with CTRL + C
5) Create a new file and past the selection with CTRL + V on a new transparent layer and remove any non-transparent layers below
6) Set the grid again
7) Use the Image --> Snap to Grid and your file is ready and clean
If you need further help comment again!
By the way, I can slice things up for you in no time if you want, just list the things you want to have in separate files :)