Very messy code that generates CS 1.6 background tiles

import time
from Tkinter import Tk
import math
import os
from tkFileDialog import askopenfilename
from PIL import Image

Tk().withdraw() # we don't want a full GUI, so keep the root window from appearing
filepath = askopenfilename() # show an "Open" dialog box and return the path to the selected file
im = Image.open(filepath)
print im.size

Width = im.size[0]
Height = im.size[1]

Columns = int(math.ceil(im.size[0]/256)) + 1
Rows = int(math.ceil(im.size[1]/256)) + 1
print str(Columns) + " columns and " + str(Rows) + " rows."

filefolder = filepath.split("/")
filefolder.pop(len(filefolder) - 1)
filefolder = "/".join(filefolder)
filename = filepath.split("/")[len(filepath.split("/")) - 1]

print(filepath)
print filefolder
print filename

Before = time.time()
os.system("convert -crop 256x256 \"" + filepath + '" "' + filefolder + '/tile_%d.tga"')
print time.time() - Before

RNS = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
CNS = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p"]

N = 0
R = 0
while R < Rows:
    C = 0
    while C < Columns:
        os.rename(filefolder + "/tile_" + str(N) + ".tga", filefolder + "/" + str(Width) + "_" + str(RNS[R]) + "_" + CNS[C] + "_loading.tga")
        N = N + 1
        C = C + 1
    R = R + 1